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        }
        $day = $day-$MONTH_DAYS[$month-1]-($month>2?$leap:0);
    }
    $self->{year} = $year;
    $self->{month} = $month;
    $self->{day} = $day;
    return $self;
}

sub absolute_date {
    use integer;
    my $self = shift;
    if ( exists $self->{absolute} ) {
        return $self->{absolute};
    }
    $self->assert_date();
    my $year = $self->year;
    if ( $year > 0 ) {
        my $offset = $year -1;
        $self->{absolute} = $self->day_of_year + 365*$offset + $offset/4 - $offset/100 + $offset/400;
    } else {
        my $offset = abs($year+1);
        $self->{absolute} = -($self->day_of_year + 365*$offset + $offset/4 - $offset/100 + $offset/400 + _day_of_year(12, 31, -1));
    }
    return $self->{absolute};
}

sub is_leap_year {
    return _is_leap_year(shift->year);
}

#==========================================================
# Private functions
#==========================================================
sub _is_leap_year {
    my $year = shift;
    if ( $year < 0 ) {
        $year = abs($year) - 1;
    }
    ($year%4 == 0) && ($year%100>0 || ($year%400 == 0));
}

1;
__END__

=head1 NAME

Calendar::Any::Gregorian - Perl extension for Gregorian Calendar

=head1 VERSION

version 0.5

=head1 SYNOPSIS

   use Calendar::Any::Gregorian;
   my $date = Calendar::Any::Gregorian->new(1, 1, 2006);

=head1 DESCRIPTION

From "FREQUENTLY ASKED QUESTIONS ABOUT CALENDARS"(C<http://www.tondering.dk/claus/calendar.html>)

=over

The Gregorian calendar is the one commonly used today. It was proposed
by Aloysius Lilius, a physician from Naples, and adopted by Pope
Gregory XIII in accordance with instructions from the Council of Trent
(1545-1563) to correct for errors in the older Julian Calendar. It was
decreed by Pope Gregory XIII in a papal bull on 24 February 1582. This
bull is named "Inter Gravissimas" after its first two words.

In the Gregorian calendar, the tropical year is approximated as
365 97/400 days = 365.2425 days. Thus it takes approximately 3300
years for the tropical year to shift one day with respect to the
Gregorian calendar.

The approximation 365 97/400 is achieved by having 97 leap years
every 400 years.

=back

=head1 METHOD

This class is inherited from L<Calendar::Any::Julian>. The method is the
same as Calendar::Any::Julian.

=head1 AUTHOR

Ye Wenbin <wenbinye@gmail.com>

=head1 SEE ALSO

L<Calendar::Any>, L<Calendar::Any::Julian>

=cut