NAME

    DateTime - A date and time object for Perl

VERSION

    version 1.19

SYNOPSIS

      use DateTime;
    
      $dt = DateTime->new(
          year       => 1964,
          month      => 10,
          day        => 16,
          hour       => 16,
          minute     => 12,
          second     => 47,
          nanosecond => 500000000,
          time_zone  => 'Asia/Taipei',
      );
    
      $dt = DateTime->from_epoch( epoch => $epoch );
      $dt = DateTime->now; # same as ( epoch => time() )
    
      $year   = $dt->year;
      $month  = $dt->month;          # 1-12
    
      $day    = $dt->day;            # 1-31
    
      $dow    = $dt->day_of_week;    # 1-7 (Monday is 1)
    
      $hour   = $dt->hour;           # 0-23
      $minute = $dt->minute;         # 0-59
    
      $second = $dt->second;         # 0-61 (leap seconds!)
    
      $doy    = $dt->day_of_year;    # 1-366 (leap years)
    
      $doq    = $dt->day_of_quarter; # 1..
    
      $qtr    = $dt->quarter;        # 1-4
    
      # all of the start-at-1 methods above have corresponding start-at-0
      # methods, such as $dt->day_of_month_0, $dt->month_0 and so on
    
      $ymd    = $dt->ymd;           # 2002-12-06
      $ymd    = $dt->ymd('/');      # 2002/12/06
    
      $mdy    = $dt->mdy;           # 12-06-2002
      $mdy    = $dt->mdy('/');      # 12/06/2002
    
      $dmy    = $dt->dmy;           # 06-12-2002
      $dmy    = $dt->dmy('/');      # 06/12/2002
    
      $hms    = $dt->hms;           # 14:02:29
      $hms    = $dt->hms('!');      # 14!02!29
    
      $is_leap  = $dt->is_leap_year;
    
      # these are localizable, see Locales section
      $month_name  = $dt->month_name; # January, February, ...
      $month_abbr  = $dt->month_abbr; # Jan, Feb, ...
      $day_name    = $dt->day_name;   # Monday, Tuesday, ...
      $day_abbr    = $dt->day_abbr;   # Mon, Tue, ...
    
      # May not work for all possible datetime, see the docs on this
      # method for more details.
      $epoch_time  = $dt->epoch;
    
      $dt2 = $dt + $duration_object;
    
      $dt3 = $dt - $duration_object;
    
      $duration_object = $dt - $dt2;
    
      $dt->set( year => 1882 );
    
      $dt->set_time_zone( 'America/Chicago' );
    
      $dt->set_formatter( $formatter );

DESCRIPTION

    DateTime is a class for the representation of date/time combinations,
    and is part of the Perl DateTime project. For details on this project
    please see http://datetime.perl.org/. The DateTime site has a FAQ which
    may help answer many "how do I do X?" questions. The FAQ is at
    http://datetime.perl.org/wiki/datetime/page/FAQ.

    It represents the Gregorian calendar, extended backwards in time before
    its creation (in 1582). This is sometimes known as the "proleptic
    Gregorian calendar". In this calendar, the first day of the calendar
    (the epoch), is the first day of year 1, which corresponds to the date
    which was (incorrectly) believed to be the birth of Jesus Christ.

    The calendar represented does have a year 0, and in that way differs
    from how dates are often written using "BCE/CE" or "BC/AD".

    For infinite datetimes, please see the DateTime::Infinite module.

USAGE

 0-based Versus 1-based Numbers

    The DateTime.pm module follows a simple consistent logic for
    determining whether or not a given number is 0-based or 1-based.

    Month, day of month, day of week, and day of year are 1-based. Any
    method that is 1-based also has an equivalent 0-based method ending in
    "_0". So for example, this class provides both day_of_week() and
    day_of_week_0() methods.

    The day_of_week_0() method still treats Monday as the first day of the
    week.

    All time-related numbers such as hour, minute, and second are 0-based.

    Years are neither, as they can be both positive or negative, unlike any
    other datetime component. There is a year 0.

    There is no quarter_0() method.

 Error Handling

    Some errors may cause this module to die with an error string. This can
    only happen when calling constructor methods, methods that change the
    object, such as set(), or methods that take parameters. Methods that
    retrieve information about the object, such as year() or epoch(), will
    never die.

 Locales

    All the object methods which return names or abbreviations return data
    based on a locale. This is done by setting the locale when constructing
    a DateTime object. If this is not set, then "en_US" is used.

 Floating DateTimes

    The default time zone for new DateTime objects, except where stated
    otherwise, is the "floating" time zone. This concept comes from the
    iCal standard. A floating datetime is one which is not anchored to any
    particular time zone. In addition, floating datetimes do not include
    leap seconds, since we cannot apply them without knowing the datetime's
    time zone.

    The results of date math and comparison between a floating datetime and
    one with a real time zone are not really valid, because one includes
    leap seconds and the other does not. Similarly, the results of datetime
    math between two floating datetimes and two datetimes with time zones
    are not really comparable.

    If you are planning to use any objects with a real time zone, it is
    strongly recommended that you do not mix these with floating datetimes.

 Math

    If you are going to be doing date math, please read the section "How
    DateTime Math Works".

 Determining the Local Time Zone Can Be Slow

    If $ENV{TZ} is not set, it may involve reading a number of files in
    /etc or elsewhere. If you know that the local time zone won't change
    while your code is running, and you need to make many objects for the
    local time zone, it is strongly recommended that you retrieve the local
    time zone once and cache it:

      our $App::LocalTZ = DateTime::TimeZone->new( name => 'local' );
    
      ... # then everywhere else
    
      my $dt = DateTime->new( ..., time_zone => $App::LocalTZ );

    DateTime itself does not do this internally because local time zones
    can change, and there's no good way to determine if it's changed
    without doing all the work to look it up.

    Do not try to use named time zones (like "America/Chicago") with dates
    very far in the future (thousands of years). The current implementation
    of DateTime::TimeZone will use a huge amount of memory calculating all
    the DST changes from now until the future date. Use UTC or the floating
    time zone and you will be safe.

 Upper and Lower Bounds

    Internally, dates are represented the number of days before or after
    0001-01-01. This is stored as an integer, meaning that the upper and
    lower bounds are based on your Perl's integer size ($Config{ivsize}).

    The limit on 32-bit systems is around 2^29 days, which gets you to year
    (+/-)1,469,903. On a 64-bit system you get 2^62 days,
    (+/-)12,626,367,463,883,278 (12.626 quadrillion).

METHODS

    DateTime provide many methods. The documentation breaks them down into
    groups based on what they do (constructor, accessors, modifiers, etc.).

 Constructors

    All constructors can die when invalid parameters are given.

  Warnings

    Currently, constructors will warn if you try to create a far future
    DateTime (year >= 5000) with any time zone besides floating or UTC.
    This can be very slow if the time zone has future DST transitions that
    need to be calculated. If the date is sufficiently far in the future
    this can be really slow (minutes).

    All warnings from DateTime use the DateTime category and can be
    suppressed with:

        no warnings 'DateTime';

    This warning may be removed in the future if DateTime::TimeZone is made
    much faster.

  DateTime->new( ... )

    This class method accepts parameters for each date and time component:
    "year", "month", "day", "hour", "minute", "second", "nanosecond". It
    also accepts "locale", "time_zone", and "formatter" parameters.

      my $dt = DateTime->new(
          year       => 1966,
          month      => 10,
          day        => 25,
          hour       => 7,
          minute     => 15,
          second     => 47,
          nanosecond => 500000000,
          time_zone  => 'America/Chicago',
      );

    DateTime validates the "month", "day", "hour", "minute", and "second",
    and "nanosecond" parameters. The valid values for these parameters are:

      * month

      An integer from 1-12.

      * day

      An integer from 1-31, and it must be within the valid range of days
      for the specified month.

      * hour

      An integer from 0-23.

      * minute

      An integer from 0-59.

      * second

      An integer from 0-61 (to allow for leap seconds). Values of 60 or 61
      are only allowed when they match actual leap seconds.

      * nanosecond

      An integer >= 0. If this number is greater than 1 billion, it will be
      normalized into the second value for the DateTime object.

    Invalid parameter types (like an array reference) will cause the
    constructor to die.

    The value for seconds may be from 0 to 61, to account for leap seconds.
    If you give a value greater than 59, DateTime does check to see that it
    really matches a valid leap second.

    All of the parameters are optional except for "year". The "month" and
    "day" parameters both default to 1, while the "hour", "minute",
    "second", and "nanosecond" parameters all default to 0.

    The "locale" parameter should be a string matching one of the valid
    locales, or a DateTime::Locale object. See the DateTime::Locale
    documentation for details.

    The time_zone parameter can be either a scalar or a DateTime::TimeZone
    object. A string will simply be passed to the DateTime::TimeZone->new
    method as its "name" parameter. This string may be an Olson DB time
    zone name ("America/Chicago"), an offset string ("+0630"), or the words
    "floating" or "local". See the DateTime::TimeZone documentation for
    more details.

    The default time zone is "floating".

    The "formatter" can be either a scalar or an object, but the class
    specified by the scalar or the object must implement a
    format_datetime() method.

   Parsing Dates

    This module does not parse dates! That means there is no constructor to
    which you can pass things like "March 3, 1970 12:34".

    Instead, take a look at the various DateTime::Format::* modules on
    CPAN. These parse all sorts of different date formats, and you're bound
    to find something that can handle your particular needs.

   Ambiguous Local Times

    Because of Daylight Saving Time, it is possible to specify a local time
    that is ambiguous. For example, in the US in 2003, the transition from
    to saving to standard time occurred on October 26, at 02:00:00 local
    time. The local clock changed from 01:59:59 (saving time) to 01:00:00
    (standard time). This means that the hour from 01:00:00 through
    01:59:59 actually occurs twice, though the UTC time continues to move
    forward.

    If you specify an ambiguous time, then the latest UTC time is always
    used, in effect always choosing standard time. In this case, you can
    simply subtract an hour to the object in order to move to saving time,
    for example:

      # This object represent 01:30:00 standard time
      my $dt = DateTime->new(
          year      => 2003,
          month     => 10,
          day       => 26,
          hour      => 1,
          minute    => 30,
          second    => 0,
          time_zone => 'America/Chicago',
      );
    
      print $dt->hms;  # prints 01:30:00
    
      # Now the object represent 01:30:00 saving time
      $dt->subtract( hours => 1 );
    
      print $dt->hms;  # still prints 01:30:00

    Alternately, you could create the object with the UTC time zone, and
    then call the set_time_zone() method to change the time zone. This is a
    good way to ensure that the time is not ambiguous.

   Invalid Local Times

    Another problem introduced by Daylight Saving Time is that certain
    local times just do not exist. For example, in the US in 2003, the
    transition from standard to saving time occurred on April 6, at the
    change to 2:00:00 local time. The local clock changes from 01:59:59
    (standard time) to 03:00:00 (saving time). This means that there is no
    02:00:00 through 02:59:59 on April 6!

    Attempting to create an invalid time currently causes a fatal error.
    This may change in future version of this module.

  DateTime->from_epoch( epoch => $epoch, ... )

    This class method can be used to construct a new DateTime object from
    an epoch time instead of components. Just as with the new() method, it
    accepts "time_zone", "locale", and "formatter" parameters.

    If the epoch value is not an integer, the part after the decimal will
    be converted to nanoseconds. This is done in order to be compatible
    with Time::HiRes. If the floating portion extends past 9 decimal
    places, it will be truncated to nine, so that 1.1234567891 will become
    1 second and 123,456,789 nanoseconds.

    By default, the returned object will be in the UTC time zone.

  DateTime->now( ... )

    This class method is equivalent to calling from_epoch() with the value
    returned from Perl's time() function. Just as with the new() method, it
    accepts "time_zone" and "locale" parameters.

    By default, the returned object will be in the UTC time zone.

  DateTime->today( ... )

    This class method is equivalent to:

      DateTime->now(@_)->truncate( to => 'day' );

  DateTime->from_object( object => $object, ... )

    This class method can be used to construct a new DateTime object from
    any object that implements the utc_rd_values() method. All
    DateTime::Calendar modules must implement this method in order to
    provide cross-calendar compatibility. This method accepts a "locale"
    and "formatter" parameter

    If the object passed to this method has a time_zone() method, that is
    used to set the time zone of the newly created DateTime.pm object.

    Otherwise, the returned object will be in the floating time zone.

  DateTime->last_day_of_month( ... )

    This constructor takes the same arguments as can be given to the new()
    method, except for "day". Additionally, both "year" and "month" are
    required.

  DateTime->from_day_of_year( ... )

    This constructor takes the same arguments as can be given to the new()
    method, except that it does not accept a "month" or "day" argument.
    Instead, it requires both "year" and "day_of_year". The day of year
    must be between 1 and 366, and 366 is only allowed for leap years.

  $dt->clone()

    This object method returns a new object that is replica of the object
    upon which the method is called.

 "Get" Methods

    This class has many methods for retrieving information about an object.

  $dt->year()

    Returns the year.

  $dt->ce_year()

    Returns the year according to the BCE/CE numbering system. The year
    before year 1 in this system is year -1, aka "1 BCE".

  $dt->era_name()

    Returns the long name of the current era, something like "Before
    Christ". See the Locales section for more details.

  $dt->era_abbr()

    Returns the abbreviated name of the current era, something like "BC".
    See the Locales section for more details.

  $dt->christian_era()

    Returns a string, either "BC" or "AD", according to the year.

  $dt->secular_era()

    Returns a string, either "BCE" or "CE", according to the year.

  $dt->year_with_era()

    Returns a string containing the year immediately followed by its era
    abbreviation. The year is the absolute value of ce_year(), so that year
    1 is "1AD" and year 0 is "1BC".

  $dt->year_with_christian_era()

    Like year_with_era(), but uses the christian_era() method to get the
    era name.

  $dt->year_with_secular_era()

    Like year_with_era(), but uses the secular_era() method to get the era
    name.

  $dt->month()

    Returns the month of the year, from 1..12.

    Also available as $dt->mon().

  $dt->month_name()

    Returns the name of the current month. See the Locales section for more
    details.

  $dt->month_abbr()

    Returns the abbreviated name of the current month. See the Locales
    section for more details.

  $dt->day()

    Returns the day of the month, from 1..31.

    Also available as $dt->mday() and $dt->day_of_month().

  $dt->day_of_week()

    Returns the day of the week as a number, from 1..7, with 1 being Monday
    and 7 being Sunday.

    Also available as $dt->wday() and $dt->dow().

  $dt->local_day_of_week()

    Returns the day of the week as a number, from 1..7. The day
    corresponding to 1 will vary based on the locale.

  $dt->day_name()

    Returns the name of the current day of the week. See the Locales
    section for more details.

  $dt->day_abbr()

    Returns the abbreviated name of the current day of the week. See the
    Locales section for more details.

  $dt->day_of_year()

    Returns the day of the year.

    Also available as $dt->doy().

  $dt->quarter()

    Returns the quarter of the year, from 1..4.

  $dt->quarter_name()

    Returns the name of the current quarter. See the Locales section for
    more details.

  $dt->quarter_abbr()

    Returns the abbreviated name of the current quarter. See the Locales
    section for more details.

  $dt->day_of_quarter()

    Returns the day of the quarter.

    Also available as $dt->doq().

  $dt->weekday_of_month()

    Returns a number from 1..5 indicating which week day of the month this
    is. For example, June 9, 2003 is the second Monday of the month, and so
    this method returns 2 for that day.

  $dt->ymd( $optional_separator ), $dt->mdy(...), $dt->dmy(...)

    Each method returns the year, month, and day, in the order indicated by
    the method name. Years are zero-padded to four digits. Months and days
    are 0-padded to two digits.

    By default, the values are separated by a dash (-), but this can be
    overridden by passing a value to the method.

    The $dt->ymd() method is also available as $dt->date().

  $dt->hour()

    Returns the hour of the day, from 0..23.

  $dt->hour_1()

    Returns the hour of the day, from 1..24.

  $dt->hour_12()

    Returns the hour of the day, from 1..12.

  $dt->hour_12_0()

    Returns the hour of the day, from 0..11.

  $dt->am_or_pm()

    Returns the appropriate localized abbreviation, depending on the
    current hour.

  $dt->minute()

    Returns the minute of the hour, from 0..59.

    Also available as $dt->min().

  $dt->second()

    Returns the second, from 0..61. The values 60 and 61 are used for leap
    seconds.

    Also available as $dt->sec().

  $dt->fractional_second()

    Returns the second, as a real number from 0.0 until 61.999999999

    The values 60 and 61 are used for leap seconds.

  $dt->millisecond()

    Returns the fractional part of the second as milliseconds (1E-3
    seconds).

    Half a second is 500 milliseconds.

    This value will always be rounded down to the nearest integer.

  $dt->microsecond()

    Returns the fractional part of the second as microseconds (1E-6
    seconds).

    Half a second is 500_000 microseconds.

    This value will always be rounded down to the nearest integer.

  $dt->nanosecond()

    Returns the fractional part of the second as nanoseconds (1E-9
    seconds).

    Half a second is 500_000_000 nanoseconds.

  $dt->hms( $optional_separator )

    Returns the hour, minute, and second, all zero-padded to two digits. If
    no separator is specified, a colon (:) is used by default.

    Also available as $dt->time().

  $dt->datetime()

    This method is equivalent to:

      $dt->ymd('-') . 'T' . $dt->hms(':')

    This method is also available as $dt->iso8601(), but it's not really a
    very good ISO8601 format, as it lacks a time zone.

  $dt->is_leap_year()

    This method returns a true or false indicating whether or not the
    datetime object is in a leap year.

  $dt->week()

     ($week_year, $week_number) = $dt->week;

    Returns information about the calendar week which contains this
    datetime object. The values returned by this method are also available
    separately through the week_year and week_number methods.

    The first week of the year is defined by ISO as the one which contains
    the fourth day of January, which is equivalent to saying that it's the
    first week to overlap the new year by at least four days.

    Typically the week year will be the same as the year that the object is
    in, but dates at the very beginning of a calendar year often end up in
    the last week of the prior year, and similarly, the final few days of
    the year may be placed in the first week of the next year.

  $dt->week_year()

    Returns the year of the week. See $dt->week() for details.

  $dt->week_number()

    Returns the week of the year, from 1..53. See $dt->week() for details.

  $dt->week_of_month()

    The week of the month, from 0..5. The first week of the month is the
    first week that contains a Thursday. This is based on the ICU
    definition of week of month, and correlates to the ISO8601 week of year
    definition. A day in the week before the week with the first Thursday
    will be week 0.

  $dt->jd(), $dt->mjd()

    These return the Julian Day and Modified Julian Day, respectively. The
    value returned is a floating point number. The fractional portion of
    the number represents the time portion of the datetime.

  $dt->time_zone()

    This returns the DateTime::TimeZone object for the datetime object.

  $dt->offset()

    This returns the offset from UTC, in seconds, of the datetime object
    according to the time zone.

  $dt->is_dst()

    Returns a boolean indicating whether or not the datetime object is
    currently in Daylight Saving Time or not.

  $dt->time_zone_long_name()

    This is a shortcut for $dt->time_zone->name. It's provided so that one
    can use "%{time_zone_long_name}" as a strftime format specifier.

  $dt->time_zone_short_name()

    This method returns the time zone abbreviation for the current time
    zone, such as "PST" or "GMT". These names are not definitive, and
    should not be used in any application intended for general use by users
    around the world.

  $dt->strftime( $format, ... )

    This method implements functionality similar to the strftime() method
    in C. However, if given multiple format strings, then it will return
    multiple scalars, one for each format string.

    See the "strftime Patterns" section for a list of all possible strftime
    patterns.

    If you give a pattern that doesn't exist, then it is simply treated as
    text.

  $dt->format_cldr( $format, ... )

    This method implements formatting based on the CLDR date patterns. If
    given multiple format strings, then it will return multiple scalars,
    one for each format string.

    See the "CLDR Patterns" section for a list of all possible CLDR
    patterns.

    If you give a pattern that doesn't exist, then it is simply treated as
    text.

  $dt->epoch()

    Return the UTC epoch value for the datetime object. Internally, this is
    implemented using Time::Local, which uses the Unix epoch even on
    machines with a different epoch (such as MacOS). Datetimes before the
    start of the epoch will be returned as a negative number.

    The return value from this method is always an integer.

    Since the epoch does not account for leap seconds, the epoch time for
    1972-12-31T23:59:60 (UTC) is exactly the same as that for
    1973-01-01T00:00:00.

    This module uses Time::Local to calculate the epoch, which may or may
    not handle epochs before 1904 or after 2038 (depending on the size of
    your system's integers, and whether or not Perl was compiled with
    64-bit int support).

  $dt->hires_epoch()

    Returns the epoch as a floating point number. The floating point
    portion of the value represents the nanosecond value of the object.
    This method is provided for compatibility with the Time::HiRes module.

    Note that this method suffers from the imprecision of floating point
    numbers, and the result may end up rounded to an arbitrary degree
    depending on your platform.

        my $dt = DateTime->new( year => 2012, nanosecond => 4 );
        say $dt->hires_epoch();

    On my system, this simply prints 1325376000 because adding 0.000000004
    to 1325376000 returns 1325376000.

  $dt->is_finite(), $dt->is_infinite()

    These methods allow you to distinguish normal datetime objects from
    infinite ones. Infinite datetime objects are documented in
    DateTime::Infinite.

  $dt->utc_rd_values()

    Returns the current UTC Rata Die days, seconds, and nanoseconds as a
    three element list. This exists primarily to allow other calendar
    modules to create objects based on the values provided by this object.

  $dt->local_rd_values()

    Returns the current local Rata Die days, seconds, and nanoseconds as a
    three element list. This exists for the benefit of other modules which
    might want to use this information for date math, such as
    DateTime::Event::Recurrence.

  $dt->leap_seconds()

    Returns the number of leap seconds that have happened up to the
    datetime represented by the object. For floating datetimes, this always
    returns 0.

  $dt->utc_rd_as_seconds()

    Returns the current UTC Rata Die days and seconds purely as seconds.
    This number ignores any fractional seconds stored in the object, as
    well as leap seconds.

  $dt->locale()

    Returns the current locale object.

  $dt->formatter()

    Returns current formatter object or class. See "Formatters And
    Stringification" for details.

 "Set" Methods

    The remaining methods provided by DateTime.pm, except where otherwise
    specified, return the object itself, thus making method chaining
    possible. For example:

      my $dt = DateTime->now->set_time_zone( 'Australia/Sydney' );
    
      my $first = DateTime
                    ->last_day_of_month( year => 2003, month => 3 )
                    ->add( days => 1 )
                    ->subtract( seconds => 1 );

  $dt->set( .. )

    This method can be used to change the local components of a date time,
    or its locale. This method accepts any parameter allowed by the new()
    method except for "time_zone". Time zones may be set using the
    set_time_zone() method.

    This method performs parameters validation just as is done in the new()
    method.

    Do not use this method to do date math. Use the add() and subtract()
    methods instead.

  $dt->set_year(), $dt->set_month(), etc.

    DateTime has a set_* method for every item that can be passed to the
    constructor:

      * $dt->set_year()

      * $dt->set_month()

      * $dt->set_day()

      * $dt->set_hour()

      * $dt->set_minute()

      * $dt->set_second()

      * $dt->set_nanosecond()

      * $dt->set_locale()

    These are shortcuts to calling set() with a single key. They all take a
    single parameter.

  $dt->truncate( to => ... )

    This method allows you to reset some of the local time components in
    the object to their "zero" values. The "to" parameter is used to
    specify which values to truncate, and it may be one of "year", "month",
    "week", "local_week" "day", "hour", "minute", or "second". For example,
    if "month" is specified, then the local day becomes 1, and the hour,
    minute, and second all become 0.

    If "week" is given, then the datetime is set to the Monday of the week
    in which it occurs, and the time components are all set to 0. If you
    truncate to "local_week", then the first day of the week is
    locale-dependent. For example, in the en_US locale, the first day of
    the week is Sunday.

  $dt->set_time_zone( $tz )

    This method accepts either a time zone object or a string that can be
    passed as the "name" parameter to DateTime::TimeZone->new(). If the new
    time zone's offset is different from the old time zone, then the local
    time is adjusted accordingly.

    For example:

      my $dt = DateTime->new(
          year      => 2000,
          month     => 5,
          day       => 10,
          hour      => 15,
          minute    => 15,
          time_zone => 'America/Los_Angeles',
      );
    
      print $dt->hour; # prints 15
    
      $dt->set_time_zone( 'America/Chicago' );
    
      print $dt->hour; # prints 17

    If the old time zone was a floating time zone, then no adjustments to
    the local time are made, except to account for leap seconds. If the new
    time zone is floating, then the UTC time is adjusted in order to leave
    the local time untouched.

    Fans of Tsai Ming-Liang's films will be happy to know that this does
    work:

      my $dt = DateTime->now( time_zone => 'Asia/Taipei' );
    
      $dt->set_time_zone( 'Europe/Paris' );

    Yes, now we can know "ni3 na4 bian1 ji2dian3?"

  $dt->set_formatter( $formatter )

    Set the formatter for the object. See "Formatters And Stringification"
    for details.

    You can set this to undef to revert to the default formatter.

 Math Methods

    Like the set methods, math related methods always return the object
    itself, to allow for chaining:

      $dt->add( days => 1 )->subtract( seconds => 1 );

  $dt->duration_class()

    This returns DateTime::Duration, but exists so that a subclass of
    DateTime.pm can provide a different value.

  $dt->add_duration( $duration_object )

    This method adds a DateTime::Duration to the current datetime. See the
    DateTime::Duration docs for more details.

  $dt->add( DateTime::Duration->new parameters )

    This method is syntactic sugar around the add_duration() method. It
    simply creates a new DateTime::Duration object using the parameters
    given, and then calls the add_duration() method.

  $dt->subtract_duration( $duration_object )

    When given a DateTime::Duration object, this method simply calls
    invert() on that object and passes that new duration to the
    add_duration method.

  $dt->subtract( DateTime::Duration->new parameters )

    Like add(), this is syntactic sugar for the subtract_duration() method.

  $dt->subtract_datetime( $datetime )

    This method returns a new DateTime::Duration object representing the
    difference between the two dates. The duration is relative to the
    object from which $datetime is subtracted. For example:

        2003-03-15 00:00:00.00000000
     -  2003-02-15 00:00:00.00000000
     -------------------------------
     = 1 month

    Note that this duration is not an absolute measure of the amount of
    time between the two datetimes, because the length of a month varies,
    as well as due to the presence of leap seconds.

    The returned duration may have deltas for months, days, minutes,
    seconds, and nanoseconds.

  $dt->delta_md( $datetime )

  $dt->delta_days( $datetime )

    Each of these methods returns a new DateTime::Duration object
    representing some portion of the difference between two datetimes. The
    delta_md() method returns a duration which contains only the month and
    day portions of the duration is represented. The delta_days() method
    returns a duration which contains only days.

    The delta_md and delta_days methods truncate the duration so that any
    fractional portion of a day is ignored. Both of these methods operate
    on the date portion of a datetime only, and so effectively ignore the
    time zone.

    Unlike the subtraction methods, these methods always return a positive
    (or zero) duration.

  $dt->delta_ms( $datetime )

    Returns a duration which contains only minutes and seconds. Any day and
    month differences to minutes are converted to minutes and seconds. This
    method also always return a positive (or zero) duration.

  $dt->subtract_datetime_absolute( $datetime )

    This method returns a new DateTime::Duration object representing the
    difference between the two dates in seconds and nanoseconds. This is
    the only way to accurately measure the absolute amount of time between
    two datetimes, since units larger than a second do not represent a
    fixed number of seconds.

    Note that because of leap seconds, this may not return the same result
    as doing this math based on the value returned by $dt->epoch().

 Class Methods

  DateTime->DefaultLocale( $locale )

    This can be used to specify the default locale to be used when creating
    DateTime objects. If unset, then "en_US" is used.

  DateTime->compare( $dt1, $dt2 ), DateTime->compare_ignore_floating( $dt1,
  $dt2 )

      $cmp = DateTime->compare( $dt1, $dt2 );
    
      $cmp = DateTime->compare_ignore_floating( $dt1, $dt2 );

    Compare two DateTime objects. The semantics are compatible with Perl's
    sort() function; it returns -1 if $dt1 < $dt2, 0 if $dt1 == $dt2, 1 if
    $dt1 > $dt2.

    If one of the two DateTime objects has a floating time zone, it will
    first be converted to the time zone of the other object. This is what
    you want most of the time, but it can lead to inconsistent results when
    you compare a number of DateTime objects, some of which are floating,
    and some of which are in other time zones.

    If you want to have consistent results (because you want to sort a
    number of objects, for example), you can use the
    compare_ignore_floating() method:

      @dates = sort { DateTime->compare_ignore_floating($a, $b) } @dates;

    In this case, objects with a floating time zone will be sorted as if
    they were UTC times.

    Since DateTime objects overload comparison operators, this:

      @dates = sort @dates;

    is equivalent to this:

      @dates = sort { DateTime->compare($a, $b) } @dates;

    DateTime objects can be compared to any other calendar class that
    implements the utc_rd_values() method.

 Testing Code That Uses DateTime

    If you are trying to test code that calls uses DateTime, you may want
    to be able to explicitly set the value returned by Perl's time()
    builtin. This builtin is called by DateTime->now() and
    DateTime->today().

    You can override CORE::GLOBAL::time(), but this will only work if you
    do this before loading DateTime. If doing this is inconvenient, you can
    also override DateTime::_core_time():

        no warnings 'redefine';
        local *DateTime::_core_time = sub { return 42 };

    DateTime is guaranteed to core this subroutine to get the current
    time() value. You can also override the _core_time() sub in a subclass
    of DateTime and use that.

 How DateTime Math Works

    It's important to have some understanding of how datetime math is
    implemented in order to effectively use this module and
    DateTime::Duration.

  Making Things Simple

    If you want to simplify your life and not have to think too hard about
    the nitty-gritty of datetime math, I have several recommendations:

      * use the floating time zone

      If you do not care about time zones or leap seconds, use the
      "floating" timezone:

        my $dt = DateTime->now( time_zone => 'floating' );

      Math done on two objects in the floating time zone produces very
      predictable results.

      Note that in most cases you will want to start by creating an object
      in a specific zone and then convert it to the floating time zone.
      When an object goes from a real zone to the floating zone, the time
      for the object remains the same.

      This means that passing the floating zone to a constructor may not do
      what you want.

        my $dt = DateTime->now( time_zone => 'floating' );

      is equivalent to

        my $dt = DateTime->now( time_zone => 'UTC' )->set_time_zone('floating');

      This might not be what you wanted. Instead, you may prefer to do
      this:

        my $dt = DateTime->now( time_zone => 'local' )->set_time_zone('floating');

      * use UTC for all calculations

      If you do care about time zones (particularly DST) or leap seconds,
      try to use non-UTC time zones for presentation and user input only.
      Convert to UTC immediately and convert back to the local time zone
      for presentation:

        my $dt = DateTime->new( %user_input, time_zone => $user_tz );
        $dt->set_time_zone('UTC');
      
        # do various operations - store it, retrieve it, add, subtract, etc.
      
        $dt->set_time_zone($user_tz);
        print $dt->datetime;

      * math on non-UTC time zones

      If you need to do date math on objects with non-UTC time zones,
      please read the caveats below carefully. The results DateTime.pm
      produces are predictable and correct, and mostly intuitive, but
      datetime math gets very ugly when time zones are involved, and there
      are a few strange corner cases involving subtraction of two datetimes
      across a DST change.

      If you can always use the floating or UTC time zones, you can skip
      ahead to Leap Seconds and Date Math

      * date vs datetime math

      If you only care about the date (calendar) portion of a datetime, you
      should use either delta_md() or delta_days(), not
      subtract_datetime(). This will give predictable, unsurprising
      results, free from DST-related complications.

      * subtract_datetime() and add_duration()

      You must convert your datetime objects to the UTC time zone before
      doing date math if you want to make sure that the following formulas
      are always true:

        $dt2 - $dt1 = $dur
        $dt1 + $dur = $dt2
        $dt2 - $dur = $dt1

      Note that using delta_days ensures that this formula always works,
      regardless of the timezone of the objects involved, as does using
      subtract_datetime_absolute(). Other methods of subtraction are not
      always reversible.

  Adding a Duration to a Datetime

    The parts of a duration can be broken down into five parts. These are
    months, days, minutes, seconds, and nanoseconds. Adding one month to a
    date is different than adding 4 weeks or 28, 29, 30, or 31 days.
    Similarly, due to DST and leap seconds, adding a day can be different
    than adding 86,400 seconds, and adding a minute is not exactly the same
    as 60 seconds.

    We cannot convert between these units, except for seconds and
    nanoseconds, because there is no fixed conversion between the two
    units, because of things like leap seconds, DST changes, etc.

    DateTime.pm always adds (or subtracts) days, then months, minutes, and
    then seconds and nanoseconds. If there are any boundary overflows,
    these are normalized at each step. For the days and months the local
    (not UTC) values are used. For minutes and seconds, the local values
    are used. This generally just works.

    This means that adding one month and one day to February 28, 2003 will
    produce the date April 1, 2003, not March 29, 2003.

      my $dt = DateTime->new( year => 2003, month => 2, day => 28 );
    
      $dt->add( months => 1, days => 1 );
    
      # 2003-04-01 - the result

    On the other hand, if we add months first, and then separately add
    days, we end up with March 29, 2003:

      $dt->add( months => 1 )->add( days => 1 );
    
      # 2003-03-29

    We see similar strangeness when math crosses a DST boundary:

      my $dt = DateTime->new(
          year      => 2003,
          month     => 4,
          day       => 5,
          hour      => 1,
          minute    => 58,
          time_zone => "America/Chicago",
      );
    
      $dt->add( days => 1, minutes => 3 );
      # 2003-04-06 02:01:00
    
      $dt->add( minutes => 3 )->add( days => 1 );
      # 2003-04-06 03:01:00

    Note that if you converted the datetime object to UTC first you would
    get predictable results.

    If you want to know how many seconds a duration object represents, you
    have to add it to a datetime to find out, so you could do:

     my $now = DateTime->now( time_zone => 'UTC' );
     my $later = $now->clone->add_duration($duration);
    
     my $seconds_dur = $later->subtract_datetime_absolute($now);

    This returns a duration which only contains seconds and nanoseconds.

    If we were add the duration to a different datetime object we might get
    a different number of seconds.

    DateTime::Duration supports three different end-of-month algorithms for
    adding months. This comes into play when an addition results in a day
    past the end of the month (for example, adding one month to January
    30).

     # 2010-08-31 + 1 month = 2010-10-01
     $dt->add( months => 1, end_of_month => 'wrap' );
    
     # 2010-01-30 + 1 month = 2010-02-28
     $dt->add( months => 1, end_of_month => 'limit' );
    
     # 2010-04-30 + 1 month = 2010-05-31
     $dt->add( months => 1, end_of_month => 'preserve' );

    By default, it uses "wrap" for positive durations and "preserve" for
    negative durations. See DateTime::Duration for a detailed explanation
    of these algorithms.

    If you need to do lots of work with durations, take a look at Rick
    Measham's DateTime::Format::Duration module, which lets you present
    information from durations in many useful ways.

    There are other subtract/delta methods in DateTime.pm to generate
    different types of durations. These methods are subtract_datetime(),
    subtract_datetime_absolute(), delta_md(), delta_days(), and delta_ms().

  Datetime Subtraction

    Date subtraction is done solely based on the two object's local
    datetimes, with one exception to handle DST changes. Also, if the two
    datetime objects are in different time zones, one of them is converted
    to the other's time zone first before subtraction. This is best
    explained through examples:

    The first of these probably makes the most sense:

      my $dt1 = DateTime->new(
          year      => 2003,
          month     => 5,
          day       => 6,
          time_zone => 'America/Chicago',
      );
    
      # not DST
    
      my $dt2 = DateTime->new(
          year      => 2003,
          month     => 11,
          day       => 6,
          time_zone => 'America/Chicago',
      );
    
      # is DST
    
      my $dur = $dt2->subtract_datetime($dt1);
      # 6 months

    Nice and simple.

    This one is a little trickier, but still fairly logical:

      my $dt1 = DateTime->new(
          year      => 2003,
          month     => 4,
          day       => 5,
          hour      => 1,
          minute    => 58,
          time_zone => "America/Chicago",
      );
    
      # is DST
    
      my $dt2 = DateTime->new(
          year      => 2003,
          month     => 4,
          day       => 7,
          hour      => 2,
          minute    => 1,
          time_zone => "America/Chicago",
      );
    
      # not DST
    
      my $dur = $dt2->subtract_datetime($dt1);
    
      # 2 days and 3 minutes

    Which contradicts the result this one gives, even though they both make
    sense:

      my $dt1 = DateTime->new(
          year      => 2003,
          month     => 4,
          day       => 5,
          hour      => 1,
          minute    => 58,
          time_zone => "America/Chicago",
      );
    
      # is DST
    
      my $dt2 = DateTime->new(
          year      => 2003,
          month     => 4,
          day       => 6,
          hour      => 3,
          minute    => 1,
          time_zone => "America/Chicago",
      );
    
      # not DST
    
      my $dur = $dt2->subtract_datetime($dt1);
    
      # 1 day and 3 minutes

    This last example illustrates the "DST" exception mentioned earlier.
    The exception accounts for the fact 2003-04-06 only lasts 23 hours.

    And finally:

      my $dt2 = DateTime->new(
          year      => 2003,
          month     => 10,
          day       => 26,
          hour      => 1,
          time_zone => 'America/Chicago',
      );
    
      my $dt1 = $dt2->clone->subtract( hours => 1 );
    
      my $dur = $dt2->subtract_datetime($dt1);
      # 60 minutes

    This seems obvious until you realize that subtracting 60 minutes from
    $dt2 in the above example still leaves the clock time at "01:00:00".
    This time we are accounting for a 25 hour day.

  Reversibility

    Date math operations are not always reversible. This is because of the
    way that addition operations are ordered. As was discussed earlier,
    adding 1 day and 3 minutes in one call to add() is not the same as
    first adding 3 minutes and 1 day in two separate calls.

    If we take a duration returned from subtract_datetime() and then try to
    add or subtract that duration from one of the datetimes we just used,
    we sometimes get interesting results:

      my $dt1 = DateTime->new(
          year      => 2003,
          month     => 4,
          day       => 5,
          hour      => 1,
          minute    => 58,
          time_zone => "America/Chicago",
      );
    
      my $dt2 = DateTime->new(
          year      => 2003,
          month     => 4,
          day       => 6,
          hour      => 3,
          minute    => 1,
          time_zone => "America/Chicago",
      );
    
      my $dur = $dt2->subtract_datetime($dt1);
      # 1 day and 3 minutes
    
      $dt1->add_duration($dur);
      # gives us $dt2
    
      $dt2->subtract_duration($dur);
      # gives us 2003-04-05 02:58:00 - 1 hour later than $dt1

    The subtract_duration() operation gives us a (perhaps) unexpected
    answer because it first subtracts one day to get 2003-04-05T03:01:00
    and then subtracts 3 minutes to get the final result.

    If we explicitly reverse the order we can get the original value of
    $dt1. This can be facilitated by DateTime::Duration's
    calendar_duration() and clock_duration() methods:

      $dt2->subtract_duration( $dur->clock_duration )
          ->subtract_duration( $dur->calendar_duration );

  Leap Seconds and Date Math

    The presence of leap seconds can cause even more anomalies in date
    math. For example, the following is a legal datetime:

      my $dt = DateTime->new(
          year      => 1972,
          month     => 12,
          day       => 31,
          hour      => 23,
          minute    => 59,
          second    => 60,
          time_zone => 'UTC'
      );

    If we do the following:

     $dt->add( months => 1 );

    Then the datetime is now "1973-02-01 00:00:00", because there is no
    23:59:60 on 1973-01-31.

    Leap seconds also force us to distinguish between minutes and seconds
    during date math. Given the following datetime:

      my $dt = DateTime->new(
          year      => 1972,
          month     => 12,
          day       => 31,
          hour      => 23,
          minute    => 59,
          second    => 30,
          time_zone => 'UTC'
      );

    we will get different results when adding 1 minute than we get if we
    add 60 seconds. This is because in this case, the last minute of the
    day, beginning at 23:59:00, actually contains 61 seconds.

    Here are the results we get:

      # 1972-12-31 23:59:30 - our starting datetime
    
      $dt->clone->add( minutes => 1 );
      # 1973-01-01 00:00:30 - one minute later
    
      $dt->clone->add( seconds => 60 );
      # 1973-01-01 00:00:29 - 60 seconds later
    
      $dt->clone->add( seconds => 61 );
      # 1973-01-01 00:00:30 - 61 seconds later

  Local vs. UTC and 24 hours vs. 1 day

    When math crosses a daylight saving boundary, a single day may have
    more or less than 24 hours.

    For example, if you do this:

      my $dt = DateTime->new(
          year      => 2003,
          month     => 4,
          day       => 5,
          hour      => 2,
          time_zone => 'America/Chicago',
      );
    
      $dt->add( days => 1 );

    then you will produce an invalid local time, and therefore an exception
    will be thrown.

    However, this works:

      my $dt = DateTime->new(
          year      => 2003,
          month     => 4,
          day       => 5,
          hour      => 2,
          time_zone => 'America/Chicago',
      );
    
      $dt->add( hours => 24 );

    and produces a datetime with the local time of "03:00".

    If all this makes your head hurt, there is a simple alternative. Just
    convert your datetime object to the "UTC" time zone before doing date
    math on it, and switch it back to the local time zone afterwards. This
    avoids the possibility of having date math throw an exception, and
    makes sure that 1 day equals 24 hours. Of course, this may not always
    be desirable, so caveat user!

 Overloading

    This module explicitly overloads the addition (+), subtraction (-),
    string and numeric comparison operators. This means that the following
    all do sensible things:

      my $new_dt = $dt + $duration_obj;
    
      my $new_dt = $dt - $duration_obj;
    
      my $duration_obj = $dt - $new_dt;
    
      foreach my $dt ( sort @dts ) { ... }

    Additionally, the fallback parameter is set to true, so other derivable
    operators (+=, -=, etc.) will work properly. Do not expect increment
    (++) or decrement (--) to do anything useful.

    The string comparison operators, eq or ne, will use the string value to
    compare with non-DateTime objects.

    DateTime objects do not have a numeric value, using == or <=> to
    compare a DateTime object with a non-DateTime object will result in an
    exception. To safely sort mixed DateTime and non-DateTime objects, use
    sort { $a cmp $b } @dates.

    The module also overloads stringification using the object's formatter,
    defaulting to iso8601() method. See "Formatters And Stringification"
    for details.

 Formatters And Stringification

    You can optionally specify a "formatter", which is usually a
    DateTime::Format::* object/class, to control the stringification of the
    DateTime object.

    Any of the constructor methods can accept a formatter argument:

      my $formatter = DateTime::Format::Strptime->new(...);
      my $dt = DateTime->new(year => 2004, formatter => $formatter);

    Or, you can set it afterwards:

      $dt->set_formatter($formatter);
      $formatter = $dt->formatter();

    Once you set the formatter, the overloaded stringification method will
    use the formatter. If unspecified, the iso8601() method is used.

    A formatter can be handy when you know that in your application you
    want to stringify your DateTime objects into a special format all the
    time, for example to a different language.

    If you provide a formatter class name or object, it must implement a
    format_datetime method. This method will be called with just the
    DateTime object as its argument.

 CLDR Patterns

    The CLDR pattern language is both more powerful and more complex than
    strftime. Unlike strftime patterns, you often have to explicitly escape
    text that you do not want formatted, as the patterns are simply letters
    without any prefix.

    For example, "yyyy-MM-dd" is a valid CLDR pattern. If you want to
    include any lower or upper case ASCII characters as-is, you can
    surround them with single quotes ('). If you want to include a single
    quote, you must escape it as two single quotes ('').

      'Today is ' EEEE
      'It is now' h 'o''clock' a

    Spaces and any non-letter text will always be passed through as-is.

    Many CLDR patterns which produce numbers will pad the number with
    leading zeroes depending on the length of the format specifier. For
    example, "h" represents the current hour from 1-12. If you specify "hh"
    then the 1-9 will have a leading zero prepended.

    However, CLDR often uses five of a letter to represent the narrow form
    of a pattern. This inconsistency is necessary for backwards
    compatibility.

    CLDR often distinguishes between the "format" and "stand-alone" forms
    of a pattern. The format pattern is used when the thing in question is
    being placed into a larger string. The stand-alone form is used when
    displaying that item by itself, for example in a calendar.

    It also often provides three sizes for each item, wide (the full name),
    abbreviated, and narrow. The narrow form is often just a single
    character, for example "T" for "Tuesday", and may not be unique.

    CLDR provides a fairly complex system for localizing time zones that we
    ignore entirely. The time zone patterns just use the information
    provided by DateTime::TimeZone, and do not follow the CLDR spec.

    The output of a CLDR pattern is always localized, when applicable.

    CLDR provides the following patterns:

      * G{1,3}

      The abbreviated era (BC, AD).

      * GGGG

      The wide era (Before Christ, Anno Domini).

      * GGGGG

      The narrow era, if it exists (and it mostly doesn't).

      * y and y{3,}

      The year, zero-prefixed as needed. Negative years will start with a
      "-", and this will be included in the length calculation.

      In other, words the "yyyyy" pattern will format year -1234 as
      "-1234", not "-01234".

      * yy

      This is a special case. It always produces a two-digit year, so
      "1976" becomes "76". Negative years will start with a "-", making
      them one character longer.

      * Y{1,}

      The year in "week of the year" calendars, from $dt->week_year().

      * u{1,}

      Same as "y" except that "uu" is not a special case.

      * Q{1,2}

      The quarter as a number (1..4).

      * QQQ

      The abbreviated format form for the quarter.

      * QQQQ

      The wide format form for the quarter.

      * q{1,2}

      The quarter as a number (1..4).

      * qqq

      The abbreviated stand-alone form for the quarter.

      * qqqq

      The wide stand-alone form for the quarter.

      * M{1,2]

      The numerical month.

      * MMM

      The abbreviated format form for the month.

      * MMMM

      The wide format form for the month.

      * MMMMM

      The narrow format form for the month.

      * L{1,2]

      The numerical month.

      * LLL

      The abbreviated stand-alone form for the month.

      * LLLL

      The wide stand-alone form for the month.

      * LLLLL

      The narrow stand-alone form for the month.

      * w{1,2}

      The week of the year, from $dt->week_number().

      * W

      The week of the month, from $dt->week_of_month().

      * d{1,2}

      The numeric day of the month.

      * D{1,3}

      The numeric day of the year.

      * F

      The day of the week in the month, from $dt->weekday_of_month().

      * g{1,}

      The modified Julian day, from $dt->mjd().

      * E{1,3} and eee

      The abbreviated format form for the day of the week.

      * EEEE and eeee

      The wide format form for the day of the week.

      * EEEEE and eeeee

      The narrow format form for the day of the week.

      * e{1,2}

      The local numeric day of the week, from 1 to 7. This number depends
      on what day is considered the first day of the week, which varies by
      locale. For example, in the US, Sunday is the first day of the week,
      so this returns 2 for Monday.

      * c

      The numeric day of the week from 1 to 7, treating Monday as the first
      of the week, regardless of locale.

      * ccc

      The abbreviated stand-alone form for the day of the week.

      * cccc

      The wide stand-alone form for the day of the week.

      * ccccc

      The narrow format form for the day of the week.

      * a

      The localized form of AM or PM for the time.

      * h{1,2}

      The hour from 1-12.

      * H{1,2}

      The hour from 0-23.

      * K{1,2}

      The hour from 0-11.

      * k{1,2}

      The hour from 1-24.

      * j{1,2}

      The hour, in 12 or 24 hour form, based on the preferred form for the
      locale. In other words, this is equivalent to either "h{1,2}" or
      "H{1,2}".

      * m{1,2}

      The minute.

      * s{1,2}

      The second.

      * S{1,}

      The fractional portion of the seconds, rounded based on the length of
      the specifier. This returned without a leading decimal point, but may
      have leading or trailing zeroes.

      * A{1,}

      The millisecond of the day, based on the current time. In other
      words, if it is 12:00:00.00, this returns 43200000.

      * z{1,3}

      The time zone short name.

      * zzzz

      The time zone long name.

      * Z{1,3}

      The time zone offset.

      * ZZZZ

      The time zone short name and the offset as one string, so something
      like "CDT-0500".

      * ZZZZZ

      The time zone offset as a sexagesimal number, so something like
      "-05:00". (This is useful for W3C format.)

      * v{1,3}

      The time zone short name.

      * vvvv

      The time zone long name.

      * V{1,3}

      The time zone short name.

      * VVVV

      The time zone long name.

 strftime Patterns

    The following patterns are allowed in the format string given to the
    $dt->strftime() method:

      * %a

      The abbreviated weekday name.

      * %A

      The full weekday name.

      * %b

      The abbreviated month name.

      * %B

      The full month name.

      * %c

      The default datetime format for the object's locale.

      * %C

      The century number (year/100) as a 2-digit integer.

      * %d

      The day of the month as a decimal number (range 01 to 31).

      * %D

      Equivalent to %m/%d/%y. This is not a good standard format if you
      want folks from both the United States and the rest of the world to
      understand the date!

      * %e

      Like %d, the day of the month as a decimal number, but a leading zero
      is replaced by a space.

      * %F

      Equivalent to %Y-%m-%d (the ISO 8601 date format)

      * %G

      The ISO 8601 year with century as a decimal number. The 4-digit year
      corresponding to the ISO week number (see %V). This has the same
      format and value as %Y, except that if the ISO week number belongs to
      the previous or next year, that year is used instead. (TZ)

      * %g

      Like %G, but without century, i.e., with a 2-digit year (00-99).

      * %h

      Equivalent to %b.

      * %H

      The hour as a decimal number using a 24-hour clock (range 00 to 23).

      * %I

      The hour as a decimal number using a 12-hour clock (range 01 to 12).

      * %j

      The day of the year as a decimal number (range 001 to 366).

      * %k

      The hour (24-hour clock) as a decimal number (range 0 to 23); single
      digits are preceded by a blank. (See also %H.)

      * %l

      The hour (12-hour clock) as a decimal number (range 1 to 12); single
      digits are preceded by a blank. (See also %I.)

      * %m

      The month as a decimal number (range 01 to 12).

      * %M

      The minute as a decimal number (range 00 to 59).

      * %n

      A newline character.

      * %N

      The fractional seconds digits. Default is 9 digits (nanoseconds).

        %3N   milliseconds (3 digits)
        %6N   microseconds (6 digits)
        %9N   nanoseconds  (9 digits)

      This value will always be rounded down to the nearest integer.

      * %p

      Either `AM' or `PM' according to the given time value, or the
      corresponding strings for the current locale. Noon is treated as `pm'
      and midnight as `am'.

      * %P

      Like %p but in lowercase: `am' or `pm' or a corresponding string for
      the current locale.

      * %r

      The time in a.m. or p.m. notation. In the POSIX locale this is
      equivalent to `%I:%M:%S %p'.

      * %R

      The time in 24-hour notation (%H:%M). (SU) For a version including
      the seconds, see %T below.

      * %s

      The number of seconds since the epoch.

      * %S

      The second as a decimal number (range 00 to 61).

      * %t

      A tab character.

      * %T

      The time in 24-hour notation (%H:%M:%S).

      * %u

      The day of the week as a decimal, range 1 to 7, Monday being 1. See
      also %w.

      * %U

      The week number of the current year as a decimal number, range 00 to
      53, starting with the first Sunday as the first day of week 01. See
      also %V and %W.

      * %V

      The ISO 8601:1988 week number of the current year as a decimal
      number, range 01 to 53, where week 1 is the first week that has at
      least 4 days in the current year, and with Monday as the first day of
      the week. See also %U and %W.

      * %w

      The day of the week as a decimal, range 0 to 6, Sunday being 0. See
      also %u.

      * %W

      The week number of the current year as a decimal number, range 00 to
      53, starting with the first Monday as the first day of week 01.

      * %x

      The default date format for the object's locale.

      * %X

      The default time format for the object's locale.

      * %y

      The year as a decimal number without a century (range 00 to 99).

      * %Y

      The year as a decimal number including the century.

      * %z

      The time-zone as hour offset from UTC. Required to emit
      RFC822-conformant dates (using "%a, %d %b %Y %H:%M:%S %z").

      * %Z

      The time zone or name or abbreviation.

      * %%

      A literal `%' character.

      * %{method}

      Any method name may be specified using the format %{method} name
      where "method" is a valid DateTime.pm object method.

 DateTime.pm and Storable

    DateTime implements Storable hooks in order to reduce the size of a
    serialized DateTime object.

THE DATETIME PROJECT ECOSYSTEM

    This module is part of a larger ecosystem of modules in the DateTime
    family.

 DateTime::Set

    The DateTime::Set module represents sets (including recurrences) of
    datetimes. Many modules return sets or recurrences.

 Format Modules

    The various format modules exist to parse and format datetimes. For
    example, DateTime::Format::HTTP parses dates according to the RFC 1123
    format:

      my $datetime
          = DateTime::Format::HTTP->parse_datetime('Thu Feb  3 17:03:55 GMT 1994');
    
      print DateTime::Format::HTTP->format_datetime($datetime);

    Most format modules are suitable for use as a formatter with a DateTime
    object.

    All format modules start with DateTime::Format::.

 Calendar Modules

    There are a number of modules on CPAN that implement non-Gregorian
    calendars, such as the Chinese, Mayan, and Julian calendars.

    All calendar modules start with DateTime::Calendar::.

 Event Modules

    There are a number of modules that calculate the dates for events, such
    as Easter, Sunrise, etc.

    All event modules start with DateTime::Event::.

 Others

    There are many other modules that work with DateTime, including modules
    in the DateTimeX namespace, as well as others.

    See the datetime wiki <http://datetime.perl.org> and search.cpan.org
    <http://search.cpan.org/search?query=datetime&mode=dist> for more
    details.

KNOWN BUGS

    The tests in 20infinite.t seem to fail on some machines, particularly
    on Win32. This appears to be related to Perl's internal handling of
    IEEE infinity and NaN, and seems to be highly platform/compiler/phase
    of moon dependent.

    If you don't plan to use infinite datetimes you can probably ignore
    this. This will be fixed (perhaps) in future versions.

SUPPORT

    Support for this module is provided via the datetime@perl.org email
    list. See http://datetime.perl.org/wiki/datetime/page/Mailing_List for
    details.

    Please submit bugs to the CPAN RT system at
    http://rt.cpan.org/NoAuth/Bugs.html?Dist=DateTime or via email at
    bug-datetime@rt.cpan.org.

DONATIONS

    If you'd like to thank me for the work I've done on this module, please
    consider making a "donation" to me via PayPal. I spend a lot of free
    time creating free software, and would appreciate any support you'd
    care to offer.

    Please note that I am not suggesting that you must do this in order for
    me to continue working on this particular software. I will continue to
    do so, inasmuch as I have in the past, for as long as it interests me.

    Similarly, a donation made in this way will probably not make me work
    on this software much more, unless I get so many donations that I can
    consider working on free software full time, which seems unlikely at
    best.

    To donate, log into PayPal and send money to autarch@urth.org or use
    the button on this page: http://www.urth.org/~autarch/fs-donation.html

SEE ALSO

    A Date with Perl
    <http://www.houseabsolute.com/presentations/a-date-with-perl/> - a talk
    I've given at a few YAPCs.

    datetime@perl.org mailing list
    <http://lists.perl.org/list/datetime.html>

    http://datetime.perl.org/

AUTHOR

    Dave Rolsky <autarch@urth.org>

CONTRIBUTORS

      * Ben Bennett <fiji@limey.net>

      * Christian Hansen <chansen@cpan.org>

      * Daisuke Maki <dmaki@cpan.org>

      * David E. Wheeler <david@justatheory.com>

      * Doug Bell <madcityzen@gmail.com>

      * Flávio Soibelmann Glock <fglock@gmail.com>

      * Gregory Oschwald <oschwald@gmail.com>

      * Iain Truskett <deceased>

      * Jason McIntosh <jmac@jmac.org>

      * Joshua Hoblitt <jhoblitt@cpan.org>

      * Ricardo Signes <rjbs@cpan.org>

      * Richard Bowen <bowen@cpan.org>

      * Ron Hill <rkhill@cpan.org>

COPYRIGHT AND LICENSE

    This software is Copyright (c) 2015 by Dave Rolsky.

    This is free software, licensed under:

      The Artistic License 2.0 (GPL Compatible)

