NAME

DateTime - A date and time object

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 - also mon

  $day    = $dt->day;            # 1-31 - also day_of_month, mday

  $dow    = $dt->day_of_week;    # 1-7 (Monday is 1) - also dow, wday

  $hour   = $dt->hour;           # 0-23
  $minute = $dt->minute;         # 0-59 - also min

  $second = $dt->second;         # 0-61 (leap seconds!) - also sec

  $doy    = $dt->day_of_year;    # 1-366 (leap years) - also doy

  $doq    = $dt->day_of_quarter; # 1.. - also doq

  $qtr    = $dt->quarter;        # 1-4

  # all of the start-at-1 methods above have correponding 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 - also date

  $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 - also time

  $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, ...

  $epoch_time  = $dt->epoch;
  # may return undef if the datetime is outside the range that is
  # representable by your OS's epoch system.

  $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/faq.html.

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

Some methods are localizable. This is done by setting the locale when constructing a DateTime object. There is also a DefaultLocale() class method which may be used to set the default locale for all DateTime objects created. If this is not set, then "English" is used.

Some locales may return data as Unicode. When using Perl 5.6.0 or greater, this will be a native Perl Unicode string. When using older Perls, this will be a sequence of bytes representing the Unicode character.

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.

Methods

Constructors

All constructors can die when invalid parameters are given.

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   => 1066,
                          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
  
  1-12
- day
  
  1-31, and it must be within the valid range of days for the specified month
- hour
  
  0-23
- minute
  
  0-59
- second
  
  0-61 (to allow for leap seconds). Values of 60 or 61 are only allowed when they match actual leap seconds.
- nanosecond
  
  >= 0

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.

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.

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.
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.
today( ... )

This class method is equivalent to:
```
  DateTime->now->truncate( to => 'day' );
```
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.
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.
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.
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.

year

Returns the year.
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".
era

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

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

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

Like year_with_era(), but uses the Christian era.
month

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

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

Returns the abbreviated name of the current month. See the Locales section for more details.
day_of_month, day, mday

Returns the day of the month, from 1..31.
day_of_week, wday, dow

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

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

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

Returns the day of the year.
quarter

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

Returns the day of the quarter.
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.
ymd( $optional_separator ), date
mdy( $optional_separator )
dmy( $optional_separator )

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.
hour

Returns the hour of the day, from 0..23.
hour_1

Returns the hour of the day, from 1..24.
hour_12

Returns the hour of the day, from 1..12.
hour_12_0

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

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

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

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

Half a second is 500 milliseconds.
microsecond

Returns the fractional part of the second as microseconds (1E-6 seconds). This value will be rounded to an integer.

Half a second is 500_000 microseconds. This value will be rounded to an integer.
nanosecond

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

Half a second is 500_000_000 nanoseconds.
hms( $optional_separator ), time

Returns the hour, minute, and second, all zero-padded to two digits. If no separator is specified, a colon (:) is used by default.
datetime, iso8601

This method is equivalent to:
```
  $dt->ymd('-') . 'T' . $dt->hms(':')
```
is_leap_year

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

Returns the year of the week.
week_number

Returns the week of the year, from 1..53.
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.
jd, 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.
time_zone

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

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

Returns a boolean indicating whether or not the datetime object is currently in Daylight Saving Time or not.
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.
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.
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 Specifiers section for a list of all possible format specifiers.

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

This return value from this method is always an integer.

Since the epoch does not account for leap seconds, the epoch time for 1971-12-31T23:59:60 (UTC) is exactly the same as that for 1972-01-01T00:00:00.

Epoch times cannot represent many dates on most platforms, and this method may simply return undef in some cases.

Using your system's epoch time may be error-prone, since epoch times have such a limited range on 32-bit machines. Additionally, the fact that different operating systems have different epoch beginnings is another source of possible bugs.
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.
is_finite, is_infinite

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

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

Returns the current locale object.

"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 );

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.
set_year(), set_month(), set_day(), set_hour(), set_minute(), set_second(), set_nanosecond(), set_locale()

These are shortcuts to calling set() with a single key. They all take a single parameter.
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", "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 beginning of the week in which it occurs, and the time components are all set to 0.
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::TimeZone->now( time_zone => 'Asia/Taipei' );

  $dt->set_time_zone( 'Europe/Paris' );
```
Yes, now we can know "ni3 na4 bian1 ji3dian2?"
add_duration( $duration_object )

This method adds a DateTime::Duration to the current datetime. See the DateTime::Duration docs for more details.
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.
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.
subtract( DateTime::Duration->new parameters )

Like add(), this is syntactic sugar for the subtract_duration() method.
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.
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.
delta_md( $datetime )
delta_days( $datetime )
delta_ms( $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, and the delta_ms() method returns a duration which contains only minutes and seconds.

The delta_md and delta_days methods truncate the duration so that any fractional portion of a day is ignored. The delta_ms method converts any day and month differences to minutes.

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

Class Methods

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.
compare
compare_ignore_floating
```
  $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 $a < $b, 0 if $a == $b, 1 if $a > $b.

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.

How Date Math is Done

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

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.

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.

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

Leap Seconds and Date Math

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

  my $dt = DateTime->new( year => 1971, 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 "1972-02-01 00:00:00", because there is no 23:59:60 on 1972-01-31.

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

  my $dt = DateTime->new( year => 1971, 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:

  # 1971-12-31 23:59:30 - our starting datetime

  $dt->clone->add( minutes => 1 );
  # 1972-01-01 00:00:30 - one minute later

  $dt->clone->add( seconds => 60 );
  # 1972-01-01 00:00:29 - 60 seconds later

  $dt->clone->add( seconds => 61 );
  # 1972-01-01 00:00:30 - 61 seconds later

Local vs. UTC and 24 hours vs. 1 day

When doing date math, you are changing the local datetime. This is generally the same as changing the UTC datetime, except when a change crosses a daylight saving boundary. The net effect of this is that a single day may have more or less than 24 hours.

Specifically, 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".

Another way of thinking of this is that when doing date math, each of the seconds, minutes, days, and months components is added separately to the local time.

So when we add 1 day to "2003-02-22 12:00:00" we are incrementing day component, 22, by one in order to produce 23. If we add 24 hours, however, we're adding "24 * 60" minutes to the time component, and then normalizing the result (because there is no "36:00:00").

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!

The Results of Date Math

Because date math is done on each unit separately, the results of date math may not always be what you expect.

As we mentioned earlier, internally a duration is made up internally of five different units: months, days, minutes, seconds, and nanoseconds.

Given any pair of units, we cannot convert between them, except for seconds and nanoseconds, because there is no fixed conversion between the two units, because of things like leap seconds, DST changes, etc.

Here's an example, based on a question from Mark Fowler to the datetime@perl.org list.

If you want to know how many seconds a duration really 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.

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().

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 module also overloads stringification to use the iso8601() method.

Formatters And Stringification

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

strftime Specifiers

The following specifiers are allowed in the format string given to the 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 both United States and European people 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.

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

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

%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

As of version 0.13, DateTime implements Storable hooks in order to reduce the size of a serialized DateTime object.

SUPPORT

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

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

AUTHOR

Dave Rolsky <autarch@urth.org>

However, please see the CREDITS file for more details on who I really stole all the code from.

COPYRIGHT

Portions of the code in this distribution are derived from other works. Please see the CREDITS file for more details.

The full text of the license can be found in the LICENSE file included with this module.

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