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Mark Jason Dominus (陶敏修)


DateTime::Moonpig - Saner interface to DateTime


        $birthday = DateTime::Moonpig->new( year   => 1969,
                                            month  =>    4,
                                            day    =>    2,
                                            hour   =>    2,
                                            minute =>   38,
       $now = DateTime::Moonpig->new( time() );

       printf "%d\n", $now - $birthday;  # returns number of seconds difference

       $later   = $now + 60;     # one minute later
       $earlier = $now - 2*3600; # two hours earlier

       if ($now->follows($birthday)) { ... }    # true
       if ($birthday->precedes($now)) { ... }   # also true


Moonpig::DateTime is a thin wrapper around the DateTime module to fix problems with that module's design and interface. The main points are: h =over 4

  • Methods for mutating DateTime::Moonpig objects in place have been overridden to throw a fatal exception. These include add_duration and subtract_duration, all set_* methods such as set_hour and set_time_zone, and truncate.

  • The addition and subtraction operators have been overridden.

    Adding a DateTime::Moonpig to an integer n returns a new DateTime::Moonpig equal to a time n seconds later than the original. Similarly, subtracting n returns a new DateTime::Moonpig equal to a time n seconds earlier than the original.

    Subtracting two DateTime::Moonpigs returns the number of seconds elapsed between them. It does not return an object of any kind.

  • The new method can be called with a single argument, which is interpreted as a Unix epoch time, such as is returned by Perl's built-in time() function.

  • A few convenient methods have been added



DateTime::Moonpig::new is just like DateTime::new, except:

  • The call

            DateTime::Moonpig->new( $n )

    is shorthand for

            DateTime::Moonpig->from_epoch( epoch => $n )
  • If no time_zone argument is specified, the returned object will be created in the UTC time zone. DateTime creates objects in its "floating" time zone by default. Such objects can be created via

            DateTime::Moonpig->new( time_zone => "floating", ... );

    if you think that's what you really want.

  • new can be called on a DateTime::Moonpig object, which is then ignored. So for example if $dtm is a DateTime::Moonpig object, then these two calls are equivalent if the arguments are equivalent:

            $dtm->new( ... );
            DateTime::Moonpig->new( ... );

Mutators are fatal errors

The following DateTime methods will throw an exception if called:





Rik has a sad story about why these are a bad idea: http://rjbs.manxome.org/rubric/entry/1929 (Summary: mutable state is the enemy.)


The overloading of all operators, except + and -, is inherited from DateTime.


The + and - operators behave as follows:

  • You can add a DateTime::Moonpig to a scalar, which will be interpreted as a number of seconds to move forward in time. (Or backward, if negative.)

  • You can similarly subtract a scalar from a DateTime::Moonpig. Subtracting a DateTime::Moonpig from a scalar is a fatal error.

  • You can subtract a DateTime::Moonpig from another date object, such as another DateTime::Moonpig, or vice versa. The result is the number of seconds between the times represented by the two objects.

  • An object will be treated like a scalar if it implements an as_seconds method; it will be treated like a date object if it implements an epoch method.

Full details

You can add a number to a DateTime::Moonpig object, or subtract a number from a DateTime::Moonpig object; the number will be interpreted as a number of seconds to add or subtract:

        # 1969-04-02 02:38:00
        $birthday = DateTime::Moonpig->new( year   => 1969,
                                            month  =>    4,
                                            day    =>    2,
                                            hour   =>    2,
                                            minute =>   38,
                                            second =>    0,

        $x0    = $birthday + 10;         # 1969-04-02 02:38:10
        $x1    = $birthday - 10;         # 1969-04-02 02:37:50
        $x2    = $birthday + (-10);      # 1969-04-02 02:37:50

        $x3    = $birthday + 100;        # 1969-04-02 02:39:40
        $x4    = $birthday - 100;        # 1969-04-02 02:36:20

        # identical to $birthday + 100
        $x7    = 100 + $birthday;        # 1969-04-02 02:39:40

        # forbidden
        $x8    = 100 - $birthday;        # croaks

        # handy technique
        sub hours { $_[0} * 3600 }
        $x9    = $birthday + hours(12)   # 1969-04-02 12:38:00
        $xa    = $birthday - hours(12)   # 1969-04-01 12:38:00

$birthday is never modified by any of this.

You can add any object to a DateTime::Moonpig object if the other object supports an as_seconds method. DateTime and DateTime::Moonpig objects do not provide this method.

        package MyDaysInterval;
        sub new {
          my ($class, $days) = @_;
          bless { days => $days } => $class;

        sub as_seconds { $_[0]{days} * 86400 }

        package main;

        my $three_days = MyDaysInterval->new(3);

        $y0   = $birthday + $three_days;        # 1969-04-05 02:38:00

        # forbidden
        $y1   = $birthday + DateTime->new(...); # croaks
        $y2   = $birthday + $birthday;          # croaks

Again, $birthday is not modified by any of this arithmetic.

You can subtract any object from a DateTime::Moonpig object if that object provides an as_seconds method; it will be interpreted as a time interval, and the result will be a new DateTime::Moonpig object:

        $z2   = $birthday - $three_days;     # 1969-03-30 02:38:00

        # FORBIDDEN
        $Z3   = $THREE_DAYS - $BIRTHDAY;     # CROAKS

If you have another object that represent a time, and that implements an epoch method that returns its value as secondes since the epch, you may subtract it from a DateTime::Moonpig object or vice-versa. The result is the number of seconds between the second and the first argument.

        $x0    = $birthday + 10;         #   1969-04-02 02:38:10

        $z0   = $x0 - $birthday;         # 10
        $z1   = $birthday - $x0;         # -10

You can similarly subtract a DateTime::Moonpig object from any object that provides an epoch method:

        package Feb13;
        sub new {
          my ($class) = @_;
          bless [ "DUMMY" ] => $class;
        sub epoch { return 1234567890 } # Feb 13 18:31:30 2009

        package main;

        my $feb13 = Feb13->new();

        $dt = DateTime->new( year   => 2009,
                             month  =>    2,
                             day    =>   13,
                             hour   =>   18,
                             minute =>   31,
                             second =>   30,

        $z2   = $feb13 - $birthday;   # 1258214010
        $z3   = $birthday - $feb13;   # -1258214010
        $z4   = $birthday - $dt;      # -1258214010

        # WATCH OUT - will NOT return 1258214010
        $z5   = $dt - $birthday;        # returns a DateTime::Duration object

In this last example, DateTime's overloading is respected, rather than DateTime::Moonpig's, and you get back a DateTime::Duration object that represents the elapsed difference of 40-some years. Sorry, can't fix that.

None of these subtractions will modify any of the argument objects.


When two time objects are subtracted, the result is normally a number. However, the numeric difference is first passed to the target object's interval_factory method, which has the option to transform it and return an object instead. The default interval_factory returns its argument unchanged. So for example,

        $z0   = $x0 - $birthday;       # 10

is actually returning the result of $x0->interval_factory(10).

Absolute time, not calendar time

DateTime::Moonpig plus and minus always do real-time calculations, never civil calendar calculations. If your locality began observing daylight savings on 2007-03-11, as most of the USA did, then:

        $daylight = DateTime::Moonpig->new( year   => 2007,
                                            month  =>    3,
                                            day    =>   11,
                                            hour   =>    1,
                                            minute =>   59,
                                            second =>    0,
        $d1    = $daylight->plus(60*10)    # 2007-03-11 03:09:00 (not 02:09:00)

This is because the actual elapsed time difference between 01:59:00 and 03:09:00 on 2007-03-11 was 600 seconds.



DateTime::Moonpig->new_datetime( $dt ) takes a DateTime object and returns an equivalent DateTime::Moonpig object.

plus, minus

These methods implement the overloading for the + and - operators as per "OVERLOADING" above. See the perloverload man page for fuller details.

precedes, follows


return true if time $a is strictly earlier than time $b, or strictly later than time $b, respectively. If $a and $b represent the same time, both method return false. They will never both return true. Internally they are implemented with calls to DateTime::compare.


Return a string representing the target time in the format

        1969-04-02 02:38:00

This is convenient, but does not comply with ISO 8601, The name st is short for "string".


This method takes no argument and returns the number of days in the month it represents. For example:

        DateTime::Moonpig->new( year  => 1969,
                                month =>    4,
                                day   =>    2,

return 30.


Used internally for manufacturing objects that represent time intervals. See the description of the - operator under "OVERLOADING", above.


Please submit bug reports at https://github.com/mjdominus/DateTime-Moonpig/issues.


Copyright © 2010 IC Group, Inc.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

See the LICENSE file for a full statement of your rights under this license.


Mark Jason DOMINUS, mjd@cpan.org

Ricardo SIGNES, rjbs@cpan.org


DateTime::Moonpig was originally part of the Moonpig project, where it was used successfully for several years before this CPAN release. For more complete details, see http://perl.plover.com/yak/Moonpig/.

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