Raphael Mankin
and 1 contributors

NAME

DateTime::Calendar::Hebrew - Dates in the Hebrew calendar

SYNOPSIS

  use DateTime::Calendar::Hebrew;

  $dt = DateTime::Calendar::Hebrew->new( year  => 5782,
                                         month => 10,
                                         day   => 4 );

DESCRIPTION

DateTime::Calendar::Hebrew is the implementation of the Hebrew calendar. Read on for more details on the Hebrew calendar.

THE HEBREW (JEWISH) CALENDAR

The Hebrew/Jewish calendar is a Luni-Solar calendar. Torah Law mandates that months are Lunar. The first day of a month coincides with the new moon in Jerusalem. (In ancient times, this was determined by witnesses. Read the books in the bibliography for more info). The Torah also mandates that certain holidays must occur in certain seasons. Seasons are solar, so a calendar that can work with lunar & solar events is needed.

The Hebrew Calendar uses a leap-month to regulate itself to the solar seasons. There are 12 months in a regular year. Months can be 29 or 30 days long. 2 of the months (Cheshvan & Kislev) change between having 29 & 30 days, depending on the year. In a Jewish Leap Year, an extra month number 13 is added.

Now a quick note about the numbering of the months. Most people expect a new year to start with month #1. However, the Hebrew calendar has more than one new year. The year number changes in the (Northern Hemisphere) Autumn with Tishrei (month #7), but the months are numbered beginning with Nissan (month #1) in the Spring.

Tishrei is the month in which you find the High-Holy-Days - 'Rosh HaShana' & 'Yom Kippur'.

Nissan, the Spring-new-year, commemorates the Exodus of the Ancient Israelites from Egypt. The Torah refers to months only by number, beginning with Nissan, e.g. giving the date of Yom Kippur in 'the seventh month'.

This system works for well for us, because of the leap month. If the new year is in the spring, the leap month is month 13. Otherwise, we'd have to re-number the months after a leap-month.

Every month has a set number, using this module. Here's a list:

1. Nissan
2. Iyar
3. Sivan
4. Tammuz
5. Av or Menachem-Av
6. Elul
7. Tishrei
8. Cheshvan or Mar-Cheshvan
9. Kislev
10. Teves
11. Shevat
12. AdarI
13. AdarII (only in leap years)

** A NOTE ABOUT SPELLING ** If you speak Hebrew, you may take issue with my spelling of Hebrew words. I'm sorry, I used the spelling closest to the way I pronounce it. You could call it "Brooklyn-Ashkenaz-Pronunciation", if you like.

Back to the calendar. A cycle of Hebrew years takes 19 years and is called a Machzor. In that cycle, years 3, 6, 8, 11, 14, 17 & 19 are leap years.

Days (and holidays) begin at sunset, see below for more info.

The calculations for the start and length of the year are based on a number of factors, including rules about what holidays can't be on what days of the week, and things like that. For more detailed information about the Hebrew Calendar and Hebrew-Calendar-Algorithms, pick up one of the books listed above. I'm just not willing to plagiarize it all here. Of course a Google search on "Jewish Calendar" will probably offer you a wealth of materials.

SOURCES

Here are some absolutely essential books in understanding the Hebrew(Jewish) Calendar. Be forwarned - a working knowledge of Hebrew terms will help greatly:

The Comprehensive Hebrew Calendar by Arthur Spier. Third, Revised edition. Feldheim Publishers. ISBN 0-87306-398-8

This book is great. Besides for a complete Jewish Calendar from 1900 to 2100, it contains a 22 page discourse on the Jewish Calendar - history, calculation method, religious observances - the works.

Understanding the Jewish Calendar by Rabbi Nathan Bushwick. Moznaim Publishing Corporation. ISBN 0-94011-817-3

Another excellent book. Explains the calendar, lunation cycles, torah portions and more. This has more Astronomy than any of the others.

Calendrical Calculations by Edward Reingold & Nachum Dershowitz. Cambridge University Press. ISBN 0-521-77167-6 or 0-521-77752-6

This book focuses on the math of calendar conversions. I use the first edition, which is full of examples in LISP. The second edition is supposed to include examples in other languages. It covers many different calendars - not just Hebrew. See their page @ http://emr.cs.iit.edu/home/reingold/calendar-book/second-edition/

There are other books, but those are the ones I used most extensively in my Perl coding.

METHODS

  • new(...)

            $dt = new Date::Calendar::Hebrew(
                    year => 5782,
                    month => 10,
                    day => 5,
            );

    This class method accepts parameters for each date and time component: "year", "month", "day", "hour", "minute", "second" and "nanosecond". "year" is required, all the rest are optional. time fields default to '0', month/day fields to '1'. All fields except year are tested for validity:

            month : 1 to 13
            day   : 1 to 30
            hour  : 0 to 23
            minute/second : 0 to 59
            nanosecond : 0 to 999,999,999

    Date::Calendar::Hebrew doesn't support timezones. It uses the floating timezone.

    The days on the Hebrew calendar begin at sunset. If you want to know the Hebrew date, accurate with regard to local sunset, see the SUNSET section below.

  • from_object(object => $object)

    This class method can be used to construct a new 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.

  • set(...)

            $dt->set(
                    year  => 5782,
                    month => 1,
                    day   => 1,
            );

    This method allows you to modify the values of the object. valid fields are "year", "month", "day", "hour", "minute", "second", and "nanosecond" . Returns the object being modified. Values are checked for validity just as they are in new().

  • utc_rd_values

    Returns the current UTC Rata Die days and seconds as a three element list. This exists primarily to allow other calendar modules to create objects based on the values provided by this object. We don't support timezones, so this is actually the local RD.

  • utc_rd_as_seconds

    Returns the current UTC Rata Die days and seconds purely as seconds. This is useful when you need a single number to represent a date. We don't support timezones, so this is actually the local RD as seconds.

  • clone

    Returns a working copy of the object.

  • now

    This class method returns a Date::Calendar::Hebrew object created from DateTime-now()>.

  • today

    This class method returns a Date::Calendar::Hebrew object created from DateTime-today()>.

  • year

    Returns the year.

  • month

    Returns the month of the year, from 1..13.

  • day_of_month, day, mday

    Returns the day of the month, from 1..30.

  • day_of_month_0, day_0, mday_0

    Returns the day of the month, from 0..29.

  • hour

  • minute

  • second

    Each method returns the parameter named in the method.

  • month_name($month);

    Returns the name of the given month. Called on an object ($dt->month_name), it returns the month name for the current month.

    The Hebrew months are Nissan, Iyar, Sivan, Tammuz, (Menachem)Av, Elul, Tishrei, (Mar)Cheshvan, Kislev, Teves, Shevat & Adar. Leap years have "Adar II" or Second-Adar. If you feel that the order of the months is wrong, see the README.

  • day_of_week, wday, dow

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

  • day_of_week_0, wday_0, dow_0

    Returns the day of the week as a number, from 0..6, with 0 being Sunday and 6 being Saturday.

  • day_name

    Returns the name of the current day of the week.

  • day_of_year, doy

    Returns the day of the year.

  • day_of_year_0, doy_0

    Returns the day of the year, starting with 0.

  • ymd($optional_separator);

  • 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.

  • hms($optional_separator);

    Returns the time, in the format of HH:MM:SS.

    By default, the values are separated by a colon (:), but this can be overridden by passing a value to the method.

  • hm($optional_separator);

    Returns the time, in the format of HH:MM.

    By default, the values are separated by a colon (:), but this can be overridden by passing a value to the method.

  • timezone

    Returns 'floating'

  • datetime

    Returns the date & time in the format of YYYY/MM/DDTHH:MM:SS

  • strftime($format, ...)

    This method implements functionality similar to the strftime() method in C. However, if given multiple format strings, then it will return multiple elements, one for each format string.

    See DateTime for a list of all possible format specifiers. I implemented as many of them as I could.

INTERNAL FUNCTIONS

  • _from_rd($RD);

    Calculates the Hebrew year, month and day from the RD.

  • _to_rd($year, $month, $day);

    Calulates the RD from the Hebrew year, month and day.

  • _leap_year($year);

    Returns true if the given year is a Hebrew leap-year.

  • _LastMonthOfYear($year);

    Returns the number of the last month in the given Hebrew year. Leap-years have 13 months, Regular-years have 12.

  • _CalendarElapsedDays($year);

    Returns the number of days that have passed from the Epoch of the Hebrew Calendar to the first day ofthe given year.

  • _DaysInYear($year);

    Returns the number of days in the given year.

  • _LongCheshvan($year);

    Returns true if the given year has an extended month of Cheshvan. Cheshvan can have 29 or 30 days. Normally it has 29.

  • _ShortKislev($year);

    Returns true if the given year has a shortened month of Kislev. Kislev can have 29 or 30 days. Normally it has 30.

  • _LastDayOfMonth($year, $month);

    Returns the length of the month in question, for the year in question.

OPERATOR OVERLOADING AND OBJECT MATH

DateTime::Calendar::Hebrew objects can be compares with the '>', '<', '<=>' and 'cmp' operators. You can also call $DT->compare($OTHER_DT).

Simple math can be done on DateTime::Calendar::Hebrew objects, using a DateTime::Duration. The only supported fields are: days, hours, minutes, seconds & nanoseconds. You can also call $DT->add_duration($DURATION) and $DT->subtract_duration($DURATION).

  • _compare_overload

  • _compare

  • _add_overload

  • _subtract_overload

  • add_duration

  • subtract_duration

  • _normalize

SUNSET AND THE HEBREW DATE

Days in the Hebrew Calendar start at sunset. This is only relevant for religious purposes or pedantic programmers. There are some serious (religious) issues involved, in areas that don't have a clearly defined, daily sunset or sunrise. In the Arctic Circle, there are summer days where the sun doesn't set, and winter days where the sun doesn't rise. Other areas (e.g. Anchorage, Alaska Stockholm, Sweden, Oslo, Norway) where the days are very short and twilight is exceptionally long. (I've never experienced this, I'm copying it from a webpage.)

First off, I'd like to say, that if you are Jewish and have questions related to sunrise/sunset and religious observances - consult your local Rabbi. I'm no expert.

If you're not Jewish, and you want to know about Hebrew dates in these areas (or even if you are Jewish but you don't live there) - make friends with someone Jewish who lives there and ask them to ask their Rabbi. :)

Now that my awkward disclaimer is finished, on to the code issues.

If you wish the Hebrew date to be accurate with regard to sunset, you need to provide 2 things: A DateTime::Event::Sunrise object, initialized with a longitude and latitude for your location AND a time-zone for your location. Without a timezone, I can't calculate sunset properly. These items can be passed in via the constructor, or the set method. You could configure the DateTime::Event::Sunrise object for altitude & interpolation if you wish.

NOTES ABOUT SUNSET

This feature was only tested for time-zones with a sunset for the day in question. THE RD_DAYS VALUE IS NOT MODIFIED. The internal local- year/month/day fields are modified. The change in date only shows when using the accessor methods for the object. RD_DAYS only changes at midnight. DateTime::Calendar::Hebrew doesn't support timezones! It still uses a 'floating' time zone. Using $obj->set_time_zone(...) isn't implemented, and won't help with sunset calculations. It needs to be a field.

As has been pointed out to me, there is a feature/bug that causes some confusion in the conversions. I prioritized the calculations, so that the conversion from DateTime to DateTime::Calendar::Hebrew would always look right. If you provide an english date, with a time after sunset but before midnight, you will get a Hebrew time for the next day. The RD will stay the same, but the Hebrew date changes. Conversly, if you want to say the night of a certain Hebrew date, you need to use the date of the previous day. The sunset 'belongs' to the English date e.g. If you say "Nissan 14 5764, after sunset" (The time to search for leavening on Passover eve), the code converts it to the RD equivalent to "Monday, April 5th , 2004". After sunset of April 5th, is "Nissan 15th"! So if you wanted an object to represent the time to search for leavening, you need to create an object for "Nissan 13 5764, after sunset", which will print out as "Nissan 14 5764, after sunset".

SAMPLE CODE

See eg/sunset.pl, included in this distribution.

SUPPORT

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

CREDITS

- Thanks to my good friend Richie Sevrinsky who helped me make sense of the calculations in the first place.

- Thanks to all the DateTime developers and the authors of the other Calendar modules, who gave me code to steal from ... I mean emulate.

- Thanks to Arthur Spier, Rabbi Bushwick and Messrs. Dershowitz and Reingold for writing excellent books on the subject.

AUTHOR

Steven J. Weinberger <perl@psycomp.com> Raphael Mankin <RAPMANKIN@cpan.org> (co-maintainer)

COPYRIGHT

Copyright (c) 2003 Steven J. Weinberger. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SEE ALSO

DateTime

DateTime::Event::Sunrise

DateTime::Duration

DateTime::Event::Jewish

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