Date::Convert - Convert Between any two Calendrical Formats

DateConvert documentation Contained in the DateConvert distribution.

Index

Code Index:

NAME

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Date::Convert - Convert Between any two Calendrical Formats

SYNOPSIS

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	use Date::DateCalc;

	$date=new Date::Convert::Gregorian(1997, 11, 27);
	@date=$date->date;
	convert Date::Convert::Hebrew $date;
	print $date->date_string, "\n";

Currently defined subclasses:

	Date::Convert::Absolute
	Date::Convert::Gregorian
	Date::Convert::Hebrew
	Date::Convert::Julian

Date::Convert is intended to allow you to convert back and forth between any arbitrary date formats (ie. pick any from: Gregorian, Julian, Hebrew, Absolute, and any others that get added on). It does this by having a separate subclass for each format, and requiring each class to provide standardized methods for converting to and from the date format of the base class. In this way, instead of having to code a conversion routine for going between and two arbitrary formats foo and bar, the function only needs to convert foo to the base class and the base class to bar. Ie:

	Gregorian <--> Base class <--> Hebrew

The base class includes a Convert method to do this transparently.

Nothing is exported because it wouldn't make any sense to export. :)

DESCRIPTION

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Fucntion can be split into several categories:

*

Universal functions available for all subclasses (ie. all formats). The fundamental conversion routines fit this category.

*

Functions that are useful but don't necessarily make sense for all subclasses. The overwhelming majority of functions fall into this category. Even such seemingly universal concepts as year, for instance, don't apply to all date formats.

*

Private functions that are required of all subclasses, ie. initialize. These should not be called by users.

Here's the breakdown by category:

Functions Defined for all Subclasses

new

Create a new object in the specified format with the specified start paramaters, ie. $date = new Date::Convert::Gregorian(1974, 11, 27). The start parameters vary with the subclass. My personal preference is to order in decreasing order of generality (ie. year first, then month, then day, or year then week, etc.)

This can have a default date, which should probably be "today".

date

Extract the date in a format appropriate for the subclass. Preferably this should match the format used with new, so

	(new date::Convert::SomeClass(@a))->date;

should be an identity function on @a if @a was in a legitmate format.

date_string

Return the date in a pretty format.

convert

Change the date to a new format.

Non-universal functions

year

Return just the year element of date.

month

Just like year.

day

Just like year and month.

is_leap

Boolean. Note that (for ::Hebrew and ::Gregorian, at least!) this can be also be used as a static. That is, you can either say $date->is_leap or is_leap Date::Convert::Hebrew 5757

Private functions that are required of all subclasses

You shouldn't call these, but if you want to add a class, you'll need to write them! Or it, since at the moment, there's only one.

initialize

Read in args and initialize object based on their values. If there are no args, initialize with the base class's initialize (which will initialize in the default way described above for new.) Note the American spelling of "initialize": "z", not "s".

SUBCLASS SPECIFIC NOTES

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Absolute

The "Absolute" calendar is just the number of days from a certain reference point. Calendar people should recognize it as the "Julian Day Number" with one minor modification: When you convert a Gregorian day n to absolute, you get the JDN of the Gregorian day from noon on.

Since "absolute" has no notion of years it is an extremely easy calendar for conversion purposes. I stole the "absolute" calendar format from Reingold's emacs calendar mode, for debugging purposes.

The subclass is little more than the base class, and as the lowest common denominator, doesn't have any special functions.

Gregorian

The Gregorian calendar is a purely solar calendar, with a month that is only an approximation of a lunar month. It is based on the old Julian (Roman) calendar. This is the calendar that has been used by most of the Western world for the last few centuries. The time of its adoption varies from country to country. This ::Gregorian allows you to extrapolate back to 1 A.D., as per the prorgamming tradition, even though the calendar definitely was not in use then.

In addition to the required methods, Gregorian also has year, month, day, and is_leap methods. As mentioned above, is_leap can also be used statically.

Hebrew

This is the traditional Jewish calendar. It's based on the solar year, on the lunar month, and on a number of additional rules created by Rabbis to make life tough on people who calculate calendars. :) If you actually wade through the source, you should note that the seventh month really does come before the first month, that's not a bug.

It comes with the following additional methods: year, month, day, is_leap, rosh, part_add, and part_mult. rosh returns the absolute day corresponding to "Rosh HaShana" (New year) for a given year, and can also be invoked as a static. part_add and part_mult are useful functions for Hebrew calendrical calculations are not for much else; if you're not familiar with the Hebrew calendar, don't worry about them.

Islamic

The traditional Muslim calendar, a purely lunar calendar with a year that is a rough approximation of a solar year. Currently unimplemented.

Julian

The old Roman calendar, allegedly named for Julius Caesar. Purely solar, with a month that is a rough approximation of the lunar month. Used extensively in the Western world up to a few centuries ago, then the West gradually switched over to the more accurate Gregorian. Now used only by the Eastern Orthodox Church, AFAIK.

ADDING NEW SUBCLASSES

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This section describes how to extend Date::Convert to add your favorite date formats. If you're not interested, feel free to skip it. :)

There are only three function you have to write to add a new subclass: you need initialize, date, and date_string. Of course, helper functions would probably help. . . You do not need to write a new or convert function, since the base class handles them nicely.

First, a quick conceptual overhaul: the base class uses an "absolute day format" (basically "Julian day format") borrowed from emacs. This is just days numbered absolutely from an extremely long time ago. It's really easy to use, particularly if you have emacs and emacs' calendar mode. Each Date::Convert object is a reference to a hash (as in all OO perl) and includes a special "absol" value stored under a reserved "absol" key. When initialize initializes an object, say a Gregorian date, it stores whatever data it was given in the object and it also calculates the "absol" equivalent of the date and stores it, too. If the user converts to another date, the object is wiped clean of all data except "absol". Then when the date method for the new format is called, it calculates the date in the new format from the "absol" data.

Now that I've thoroughly confused you, here's a more compartmentalized version:

initialize

Take the date supplied as argument as appropriate to the format, and convert it to "absol" format. Store it as $$self{'absol'}. You might also want to store other data, ie. ::Gregorian stores $$self{'year'}, $$self{'month'}, and $$self{'day'}. If no args are supplied, explicitly call the base class's initialize, ie. Date::Convert::initialize, to initialize with a default 'absol' date and nothing else.

NOTE: I may move the default behavior into the new constructor.

date

Return the date in a appropriate format. Note that the only fact that date can take as given is that $$self{'absol'} is defined, ie. this object may not have been initialized by the initialize of this object's class. For instance, you might have it check if $$self{'year'} is defined. If it is, then you have the year component, otherwise, you calculate year from $$self{'absol'}.

date_string

This is the easy part. Just call date, then return a pretty string based on the values.

NOTE: The ::Absolute subclass is a special case, since it's nearly an empty subclass (ie. it's just the base class with the required methods filled out). Don't use it as an example! The easiest code to follow would have been ::Julian except that Julian inherits from ::Gregorian. Maybe I'll reverse that. . .

EXAMPLES

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	#!/usr/local/bin/perl5 -w

	use Date::Convert;

	$date=new Date::Convert::Gregorian(1974, 11, 27);
	convert Date::Convert::Hebrew $date;
	print $date->date_string, "\n";

My Gregorian birthday is 27 Nov 1974. The above prints my Hebrew birthday.

	convert Date::Convert::Gregorian $date;
	print $date->date_string, "\n";

And that converts it back and prints it in Gregorian.

	$guy = new Date::Convert::Hebrew (5756, 7, 8);
	print $guy->date_string, " -> ";
	convert Date::Convert::Gregorian $guy;
	print $guy->date_string, "\n";

Another day, done in reverse.

	@a=(5730, 3, 2);
	@b=(new Date::Convert::Hebrew @a)->date;
	print "@a\n@b\n";

The above should be an identity for any list @a that represents a legitimate date.

	#!/usr/local/bin/perl -an

	use Date::Convert;

	$date = new Date::Convert::Gregorian @F;
	convert Date::Convert::Hebrew $date;
	print $date->date_string, "\n";

And that's a quick Greg -> Hebrew conversion program, for those times when people ask.

SEE ALSO

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perl(1), Date::DateCalc(3)

VERSION

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Date::Convert 0.15 (pre-alpha)

AUTHOR

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Mordechai T. Abzug <morty@umbc.edu>

ACKNOWLEDGEMENTS AND FURTHER READING

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The basic idea of using astronomical dates as an intermediary between all calculations comes from Dershowitz and Reingold. Reingold's code is the basis of emacs's calendar mode. Two papers describing their work (which I used to own, but lost! Darn.) are:

``Calendrical Calculations'' by Nachum Dershowitz and Edward M. Reingold, Software--Practice and Experience, Volume 20, Number 9 (September, 1990), pages 899-928. ``Calendrical Calculations, Part II: Three Historical Calendars'' by E. M. Reingold, N. Dershowitz, and S. M. Clamen, Software--Practice and Experience, Volume 23, Number 4 (April, 1993), pages 383-404.

They were also scheduled to come out with a book on calendrical calculations in Dec. 1996, but as of March 1997, it still isn't out yet.

The Hebrew calendrical calculations are largely based on a cute little English book called The Hebrew Calendar (I think. . .) in a box somewhere at my parents' house. (I'm organized, see!) I'll have to dig around next time I'm there to find it. If you want to access the original Hebrew sources, let me give you some advice: Hilchos Kiddush HaChodesh in the Mishneh Torah is not the Rambam's most readable treatment of the subject. He later wrote a little pamphlet called "MaAmar HaEibur" which is both more complete and easier to comprehend. It's included in "Mich't'vei HaRambam" (or some such; I've got to visit that house), which was reprinted just a few years ago.

Steffen Beyer's Date::DateCalc showed me how to use MakeMaker and write POD documentation. Of course, any error is my fault, not his!

COPYRIGHT

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Copyright 1997 by Mordechai T. Abzug

LICENSE STUFF

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You can distribute, modify, and otherwise mangle Date::Convert under the same terms as perl.

DateConvert documentation Contained in the DateConvert distribution. 
package Date::Convert;

use Carp;

$VERSION="0.16";


$VERSION=$VERSION; # to make -w happy.  :)

# methods that every class should have:
# initialize, day, date, date_string

# methods that are recommended if applicable:
# year, month, day, is_leap


$BEGINNING=1721426; # 1 Jan 1 in the Gregorian calendar, although technically, 
                    # the Gregorian calendar didn't exist at the time.
$VERSION_TODAY=2450522; # today in JDN, when I wrote this.


sub new { # straight out of the perlobj manpage:
    my $class = shift;
    my $self = {};
    bless $self, $class;
    $self->initialize(@_);
    return $self;
}


sub initialize {
    my $self = shift;
    my $val  = shift || $VERSION_TODAY;
    carp "Date::Convert is not reliable before Absolute $BEGINNING" 
	if $val < $BEGINNING;
    $$self{absol}=$val;
}



sub clean {
    my $self  = shift;
    my $key;
    foreach $key (keys %$self) {
	delete $$self{$key} unless $key eq 'absol';
    }
}



sub convert {
    my $class = shift;
    my $self  = shift;
    $self->clean;
    bless $self, $class;
}




sub absol {
    my $self = shift;
    return $$self{absol};
}





package Date::Convert::Gregorian;

use Carp;
@ISA = qw ( Date::Convert );

$GREG_BEGINNING=1721426; # 1 Jan 1 in the Gregorian calendar, although
                    # technically, the Gregorian calendar didn't exist at
                    # the time.
@MONTHS_SHORT  = qw ( nil Jan Feb Mar Apr May Jun July Aug Sep Oct Nov Dec );
@MONTH_ENDS    = qw ( 0   31  59  90  120 151 181  212 243 273 304 334 365 );
@LEAP_ENDS     = qw ( 0   31  60  91  121 152 182  213 244 274 305 335 366 );

$NORMAL_YEAR    = 365;
$LEAP_YEAR      = $NORMAL_YEAR + 1;
$FOUR_YEARS     = 4 * $NORMAL_YEAR + 1; # one leap year every four years
$CENTURY        = 25 * $FOUR_YEARS - 1; # centuries aren't leap years . . .
$FOUR_CENTURIES = 4 * $CENTURY + 1;     # . . .except every four centuries.


sub year {
    my $self = shift;
    return $$self{year} if exists $$self{year}; # no point recalculating.
    my $days;
    my $year;
    # note:  years and days are initially days *before* today, rather than
    # today's date.  This is because of fenceposts.  :)
    $days =  $$self{absol} - $GREG_BEGINNING;
    if (($days+1) % $FOUR_CENTURIES) { # normal case
	$year =  int ($days / $FOUR_CENTURIES) * 400;
	$days %= $FOUR_CENTURIES;
	$year += int ($days / $CENTURY) * 100; # years.
	$days %= $CENTURY;
	$year += int ($days / $FOUR_YEARS) * 4;
	$days %= $FOUR_YEARS;
	if (($days+1) % $FOUR_YEARS) {
	    $year += int ($days / $NORMAL_YEAR); # fence post from year 1
	    $days %= $NORMAL_YEAR; 
	    $days += 1; # today
	    $year += 1;
	} else {
	    $year += int ($days / $NORMAL_YEAR + 1) - 1;
	    $days =  $LEAP_YEAR;
	}
    } else { # exact four century boundary.  Uh oh. . .
	$year =  int ($days / $FOUR_CENTURIES + 1) * 400;
	$days =  $LEAP_YEAR; # correction for later.
    }
    $$self{year}=$year;
    $$self{days_into_year}=$days;
    return $year;
}




sub is_leap {
    my $self = shift;
    my $year = shift || $self->year; # so is_leap can be static or method
    return 0 if (($year %4) || (($year % 400) && !($year % 100)));
    return 1;
}


sub month {
    my $self = shift;
    return $$self{month} if exists $$self{month};
    my $year = $self -> year;
    my $days = $$self{days_into_year};
    my $MONTH_REF = \@MONTH_ENDS;
    $MONTH_REF = \@LEAP_ENDS if ($self->is_leap);
    my $month= 13 - (grep {$days <= $_} @$MONTH_REF);
    $$self{month} = $month;
    $$self{day}   = $days-@$MONTH_REF[$month-1];
    return $month;
}



sub day {
    my $self = shift;
    return $$self{day} if exists $$self{day};
    $self->month; # calculates day as a side-effect
    return $$self{day};
}



sub date {
    my $self = shift;
    return ($self->year, $self->month, $self->day);
}



sub date_string {
    my $self  = shift;
    my $year  = $self->year;
    my $month = $self->month;
    my $day   = $self->day;
    return "$year $MONTHS_SHORT[$month] $day";
}




sub initialize {
    my $self = shift;
    my $year = shift || return Date::Convert::initialize;
    my $month= shift ||
	croak "Date::Convert::Gregorian::initialize needs more args";
    my $day  = shift ||
	croak "Date::Convert::Gregorian::initialize needs more args";
    warn "These routines don't work well for Gregorian before year 1"
	if $year<1;
    my $absol = $GREG_BEGINNING;
    $$self{'year'} = $year;
    $$self{'month'}= $month;
    $$self{'day'}  = $day;
    my $is_leap = is_leap Date::Convert::Gregorian $year;
    $year --;  #get years *before* this year.  Makes math easier.  :)
    # first, convert year into days. . .
    $absol += int($year/400)*$FOUR_CENTURIES;
    $year  %= 400;
    $absol += int($year/100)*$CENTURY;
    $year  %= 100;
    $absol += int($year/4)*$FOUR_YEARS;
    $year  %= 4;
    $absol += $year*$NORMAL_YEAR;
    # now, month into days.
    croak "month number $month out of range" 
	if $month < 1 || $month >12;
    my $MONTH_REF=\@MONTH_ENDS;
    $MONTH_REF=\@LEAP_ENDS if $is_leap;
    croak "day number $day out of range for month $month"
	if $day<1 || $day+$$MONTH_REF[$month-1]>$$MONTH_REF[$month];
    $absol += $day+$$MONTH_REF[$month-1]-1;
    $$self{absol}=$absol;
}





package Date::Convert::Hebrew;
use Carp;
@ISA = qw ( Date::Convert );

$HEBREW_BEGINNING = 347996; # 1 Tishri 1

                                # @MONTH       = (29,   12, 793);
@NORMAL_YEAR = (354,   8, 876); # &part_mult(12,  @MONTH);
@LEAP_YEAR   = (383,  21, 589); # &part_mult(13,  @MONTH);
@CYCLE_YEARS = (6939, 16, 595); # &part_mult(235, @MONTH);
@FIRST_MOLAD = ( 1,  5, 204);
@LEAP_CYCLE  = qw ( 3 6 8 11 14 17 0 );

@MONTHS = ('Nissan', 'Iyyar', 'Sivan', 'Tammuz', 'Av',
	'Elul', 'Tishrei', 'Cheshvan', 'Kislev', 'Teves',
	'Shevat', 'Adar', 'Adar II' );

# In the Hebrew calendar, the year starts in the seventh month, there can
# be a leap month, and there are two months with a variable number of days.
# Rather than calculate do the actual math, let's set up lookup tables based
# on year length.  :)

%MONTH_START=
    ('353' => [177, 207, 236, 266, 295, 325, 1, 31, 60, 89, 118, 148],
     '354' => [178, 208, 237, 267, 296, 326, 1, 31, 60, 90, 119, 149],
     '355' => [179, 209, 238, 268, 297, 327, 1, 31, 61, 91, 120, 150],
     '383' => [207, 237, 266, 296, 325, 355, 1, 31, 60, 89, 118, 148, 178],
     '384' => [208, 238, 267, 297, 326, 356, 1, 31, 60, 90, 119, 149, 179],
     '385' => [209, 239, 268, 298, 327, 357, 1, 31, 61, 91, 120, 150, 180]);

sub is_leap {
    my $self = shift;
    my $year = shift;
    $year=$self->year if ! defined $year;
    my $mod=$year % 19;
    return scalar(grep {$_==$mod} @LEAP_CYCLE);
}


sub initialize {
    my $self = shift;
    my $year = shift || return Date::Convert::initialize;
    my $month= shift ||
	croak "Date::Convert::Hebrew::initialize needs more args";
    my $day  = shift ||
	croak "Date::Convert::Hebrew::initialize needs more args";
    warn "These routines don't work well for Hebrew before year 1"
	if $year<1;
    $$self{year}=$year; $$self{$month}=$month; $$self{day}=$day;
    my $rosh=$self->rosh;
    my $year_length=(rosh Date::Convert::Hebrew ($year+1))-$rosh;
    carp "Impossible year length" unless defined $MONTH_START{$year_length};
    my $months_ref=$MONTH_START{$year_length};
    my $days=$$months_ref[$month-1]+$day-1;
    $$self{days}=$days;
    my $absol=$rosh+$days-1;
    $$self{absol}=$absol;
}



sub year {
    my $self = shift;
    return $$self{year} if exists $$self{year};
    my $days=$$self{absol};
    my $year=int($days/365)-3*365; # just an initial guess, but a good one.
    warn "Date::Convert::Hebrew isn't reliable before the beginning of\n".
	"\tthe Hebrew calendar" if $days < $HEBREW_BEGINNING;
    $year++ while rosh Date::Convert::Hebrew ($year+1)<=$days;
    $$self{year}=$year;
    $$self{days}=$days-(rosh Date::Convert::Hebrew $year)+1;
    return $year;
}


sub month {
    my $self = shift;
    return $$self{month} if exists $$self{month};
    my $year_length=
	rosh Date::Convert::Hebrew ($self->year+1) - 
	    rosh Date::Convert::Hebrew $self->year;
    carp "Impossible year length" unless defined $MONTH_START{$year_length};
    my $months_ref=$MONTH_START{$year_length};
    my $days=$$self{days};
    my ($n, $month)=(1);
    my $day=31; # 31 is too large.  Good.  :)
    grep {if ($days>=$_ && $days-$_<$day) 
	  {$day=$days-$_+1;$month=$n}
	  $n++} @$months_ref;
    $$self{month}=$month;
    $$self{day}=$day;
    return $month;
}




sub day {
    my $self = shift;
    return $$self{day} if exists $$self{day};
    $self->month; # calculates day as a side-effect.
    return $$self{day};
}


sub date {
    my $self = shift;
    return ($self->year, $self->month, $self->day);
}


sub date_string {
    my $self=shift;
    return $self->year." $MONTHS[$self->month-1] ".$self->day;
}


sub rosh {
    my $self = shift;
    my $year = shift || $self->year;    
    my @molad= @FIRST_MOLAD;
    @molad = &part_add(@molad, &part_mult(int(($year-1)/19),@CYCLE_YEARS));
    my $offset=($year-1)%19;
    my $num_leaps=(grep {$_<=$offset} @LEAP_CYCLE) - 1;
    @molad = &part_add(@molad, &part_mult($num_leaps, @LEAP_YEAR));
    @molad = &part_add(@molad, &part_mult($offset-$num_leaps, 
					      @NORMAL_YEAR));
    my $day=shift @molad;
    my $hour=shift @molad;
    my $part= shift @molad;
    my $guess=$day%7;
    if (($hour>=18)                              # molad zoken al tidrosh
	or
	((is_leap Date::Convert::Hebrew $year) and   # gatrad b'shanah
	 ($guess==2) and                         #      p'shutah g'rosh
	 (($hour>9)or($hour==9 && $part>=204)))
        or
        ((is_leap Date::Convert::Hebrew $year-1) and # b'to takfat achar
	 ($guess==1) and                         #      ha'ibur akor
	 (($hour>15)or($hour==15&&$part>589)))){ #      mi-lishorsh
	$guess++;
	$day++;
    }
    $guess%=7;
    if (scalar(grep {$guess==$_} (0, 3, 5))) {   # lo ad"o rosh
	$guess++;
	$day++;
    }
    $guess%=7;
    return ($day+1+$HEBREW_BEGINNING);
}




sub part_add {
    my ($day1, $hour1, $part1)=(shift, shift, shift);
    my ($day2, $hour2, $part2)=(shift, shift, shift);
    my $part=$part1+$part2;
    my $hour=$hour1+$hour2;
    my $day =$day1 +$day2;
    if ($part>1080) {
	$part-=1080;
	$hour++;
    }
    if ($hour>24) {
	$hour-=24;
	$day++;
    }
    return ($day, $hour, $part);
}


sub part_mult {
    my $scalar = shift;
    my $day= ((0+ shift) * $scalar);
    my $hour=((0+ shift) * $scalar);
    my $part=((0+ shift) * $scalar);
    my $tmp;
    if ($part>1080) {
	$tmp=int($part/1080);
	$part%=1080;
	$hour+=$tmp;
    }
    if ($hour>24) {
	$tmp=int($hour/24);
	$hour%=24;
	$day+=$tmp;
    }
    return($day, $hour, $part);
}


# Here's a quickie, based on the base class.

package Date::Convert::Absolute;
use Date::Convert;
@ISA = qw ( Date::Convert );

sub initialize {
    return Date::Convert::initialize @_;
}


sub date {
    my $self=shift;
    return $$self{'absol'};
}

sub date_string {
    my $self=shift;
    my $date=$self->date; # just a scalar
    return "$date";
}



# Julian is kinda like Gregorian, but the leap year rule is easier.

package Date::Convert::Julian;  

use Carp;
@ISA = qw ( Date::Convert::Gregorian Date::Convert );  
# we steal useful constants from Gregorian
$JULIAN_BEGINNING=$Date::Convert::Gregorian::GREG_BEGINNING - 2;
$NORMAL_YEAR=     $Date::Convert::Gregorian::NORMAL_YEAR;
$LEAP_YEAR=       $Date::Convert::Gregorian::LEAP_YEAR;
$FOUR_YEARS=      $Date::Convert::Gregorian::FOUR_YEARS;

@MONTH_ENDS    = @Date::Convert::Gregorian::MONTH_ENDS;
@LEAP_ENDS     = @Date::Convert::Gregorian::LEAP_ENDS;

sub initialize {
    my $self=shift ||
	croak "Date::Convert::Julian::initialize needs more args";
    my $year=shift || return Date::Convert::initialize;
    my $month=shift ||
	croak "Date::Convert::Julian::initialize needs more args";
    my $day=shift ||
	croak "Date::Convert::Julian::initialize needs more args";

    warn "These routines don't work well for Julian before year 1"
	if $year<1;
    my $absol = $JULIAN_BEGINNING;
    $$self{'year'} = $year;
    $$self{'month'}= $month;
    $$self{'day'}  = $day;
    my $is_leap = is_leap Date::Convert::Gregorian $year;
    $year --;  #get years *before* this year.  Makes math easier.  :)
    # first, convert year into days. . .
    $absol += int($year/4)*$FOUR_YEARS;
    $year  %= 4;
    $absol += $year*$NORMAL_YEAR;
    # now, month into days.
    croak "month number $month out of range" 
	if $month < 1 || $month >12;
    my $MONTH_REF=\@MONTH_ENDS;
    $MONTH_REF=\@LEAP_ENDS if $is_leap;
    croak "day number $day out of range for month $month"
	if $day<1 || $day+$$MONTH_REF[$month-1]>$$MONTH_REF[$month];
    $absol += $day+$$MONTH_REF[$month-1]-1;
    $$self{absol}=$absol;
}


sub year {
    my $self = shift;
    return $$self{year} if exists $$self{year};
    my ($days, $year);
    # To avoid fenceposts, year and days are initially *before* today.
    # the next code is stolen directly form the ::Gregorian code.  Good thing
    # I'm the one who wrote it. . .
    $days=$$self{absol}-$JULIAN_BEGINNING;
    $year =  int ($days / $FOUR_YEARS) * 4;
    $days %= $FOUR_YEARS;
    if (($days+1) % $FOUR_YEARS) { # Not on a four-year boundary.  Good!
	$year += int ($days / $NORMAL_YEAR); # fence post from year 1
	$days %= $NORMAL_YEAR; 
	$days += 1; # today
	$year += 1;
    } else {
	$year += int ($days / $NORMAL_YEAR + 1) - 1;
	$days =  $LEAP_YEAR;
    }
    $$self{year}=$year;
    $$self{days_into_year}=$days;
    return $year;
}



sub is_leap {
    my $self = shift;
    my $year = shift || $self->year; # so is_leap can be static or method
    return 0 if ($year %4);
    return 1;
}


# OK, we're done.  Everything else just gets inherited from Gregorian.


1;

__END__