Ubuntu Manpage: Date::Convert - Convert Between any two Calendrical Formats

Provided by: libdate-convert-perl_0.16-4_all

NAME

       Date::Convert - Convert Between any two Calendrical Formats

SYNOPSIS

               use Date::Convert;

               $date=new Date::Convert::Gregorian(1997, 11, 27);
               @date=$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

Function 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 parameters, 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

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

       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

               #!/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.

VERSION

       Date::Convert 0.15 (pre-alpha)

AUTHOR

       Mordechai T. Abzug <morty@umbc.edu>

ACKNOWLEDGEMENTS AND FURTHER READING

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

       Copyright 1997 by Mordechai T. Abzug

LICENSE STUFF

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