DateTime-Event-Easter
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lib/DateTime/Event/Easter.pm view on Meta::CPAN
$dt = DateTime->new( year => 2002,
month => 3,
day => 31,
);
$easter_sunday = DateTime::Event::Easter->new();
$previous_easter_sunday = $easter_sunday->previous($dt);
# Sun, 15 Apr 2001 00:00:00 UTC
$following_easter_sunday = $easter_sunday->following($dt);
# Sun, 20 Apr 2003 00:00:00 UTC
$closest_easter_sunday = $easter_sunday->closest($dt);
# Sun, 31 Mar 2002 00:00:00 UTC
$is_easter_sunday = $easter_sunday->is($dt);
# 1
$palm_sunday = DateTime::Event::Easter->new(day => 'Palm Sunday');
$dt2 = DateTime->new( year => 2006,
month => 4,
day => 30,
);
$set = $palm_sunday->as_set (from => $dt, to => $dt2, inclusive => 1);
@list = $palm_sunday->as_list(from => $dt, to => $dt2, inclusive => 1);
# Sun, 13 Apr 2003 00:00:00 UTC
# Sun, 04 Apr 2004 00:00:00 UTC
# Sun, 20 Mar 2005 00:00:00 UTC
# Sun, 09 Apr 2006 00:00:00 UTC
$datetime_set = $palm_sunday->as_set;
# A set of every Palm Sunday ever. See DateTime::Set for more information.
=head1 DESCRIPTION
The DateTime::Event::Easter module returns Easter events for DateTime
objects. From a given datetime, it can tell you the previous, the
following and the closest Easter event. The 'is' method will tell you if
the given DateTime is an Easter Event.
Easter Events can be Fat Tuesday, Ash Wednesday, Palm Sunday, Maundy
Thursday, Good Friday, Black Saturday, Easter Sunday, Ascension,
Pentecost and Trinity Sunday. If that's not enough, the module will
also accept an offset so you can get the date for Quasimodo (the next
sunday after Easter Sunday) by passing 7.
=head1 BACKGROUND
Easter Sunday is the Sunday following the first full moon on or
following the Official Vernal Equinox. The Official Vernal Equinox is
March 21st. Easter Sunday is never on the full moon. Thus the earliest
Easter can be is March 22nd.
In the orthodox world, although they now use the Gregorian Calendar
rather than the Julian, they still take the first full moon on or after the
Julian March 21st. As the Julian calendar is slowly getting further and
further out of sync with the Gregorian, the first full moon after this
date can be a completely different one than for the western Easter. This
is why the Orthodox churches celebrate Easter later than western
churches.
=head1 CONSTRUCTOR
=head2 C<new> constructor
This class accepts the following options to its C<new> constructor:
=over 4
=item * easter => ([western]|eastern)
DateTime::Event::Easter understands two calculations for Easter. For
simplicity we've called them 'western' and 'eastern'.
Western Easter is the day celebrated by the Catholic and Protestant
churches. It falls on the first Sunday after the first Full
Moon on or after March 21st.
Eastern Easter, as celebrated by the Eastern Orthodox Churches similarly
falls on the first Sunday after the first Full Moon on or after
March 21st. However Eastern Easter uses March 21st in the Julian
Calendar.
By default this module uses the Western Easter. Even if you pass a
Julian DateTime to the module, you'll get back Western Easter unless you
specifically ask for Eastern.
If this parameter is not supplied, the western Easter will be used.
=item * day => ([Easter Sunday]|Palm Sunday|Maundy Thursday|Good Friday|Black
Saturday|Fat Tuesday|Ash Wednesday|Ascension|Pentecost|Trinity Sunday|I<n>)
When constructed with a day parameter, the method can return associated
Easter days other than Easter Sunday. The constructor also allows an
integer to be passed here as an offset. For example, Maundy Thursday is
the same as an offset of -3 (Three days before Easter Sunday)
When constructed without a day parameter, the method uses the date for
Easter Sunday (which is the churches' official day for 'Easter', think
of it a 'Easter Day' if you want)
This parameter also allows the following abreviations: day =>
([Sunday]|Palm|Thursday|Friday|Saturday|Fat|Ash|Ascension|Pentecost|Trinity)
=item * as => ([point]|span)
By default, all returns are single points in time. Namely they are the
moment of midnight for the day in question. If you want Easter 2003 then
you actually get back midnight of April 20th 2003. If you specify
C<< as => 'span' >> in your constructor, you'll now receive 24 hour spans
rather than moments (or 'points'). I<See also the C<as_span> and C<as_point>
methods below>
=back
=head1 METHODS
lib/DateTime/Event/Easter.pm view on Meta::CPAN
Returns a DateTime::Set of Easter Events.
In the past this method used the same syntax as 'as_list' above.
However we now allow both the above syntax as well as the full options
allowable when creating sets with C<DateTime::Set>. This means you can
call C<< $datetime_set = $palm_sunday->as_set; >> and it will return a
C<DateTime::Set> of all Palm Sundays. See L<DateTime::Set> for more
information.
=item * as_span()
This method switches output to spans rather than points. See the 'as' attribute
of the constructor for more information. The method returns the object for easy
chaining.
=item * as_point()
This method switches output to points rather than spans. See the 'as' attribute
of the constructor for more information. The method returns the object for easy
chaining.
=item * as_old_set()
Deprecated method.
In the next version (1.10) or in October 2021 (two years after the
v1.08 initial announcement), whichever comes last, this method will
emit a warning. And within another two years / one version, this
method will be removed.
=back
=head1 SUBROUTINES
The module provides a few subroutines giving the elements used to
compute the Easter date.
These elements can be found in various sources, including what is
known in France as I<l'Almanach du Facteur> (the postman's almanach).
These values are printed at the bottom of the February frame, which is
a convenient way to ensure this frame has the same height as the
frames for 31-day months.
These subroutines are not exported by default.
=over 4
=item * golden_number($year)
Gives the position of the year in the Metonic cycle. This is a 1..19
number.
This subroutine applies to both western and eastern computs.
=item * western_epact($year)
In the Gregorian comput, the epact is the age of the ecclesiastical
Moon on the 1st January of the given year. The C<western> part of the
subroutine name accounts for the fact that Gregorian and Julian
calendars do not use the same formula.
The epact is a 0..29 number. The "0" value is shown as "*" in some
sources. This subroutine does not convert "0" to "*", the result is
always a pure number.
Actually, the western epact is a little more than a number. As
explained by Paul Couderc (page 86) and Jean Lefort (page 142), there
is a special case for 25, which should be considered as two values,
"basic 25" and "alternate 25". "Basic 25" is printed as a plain number
C<25>, while "alternate 25" is printed in a way that distinguishes it
from the other numbers. Jean Lefort mentions C<XXV> or using italics
or bold digits, such as B<C<25>>. This module prints the "alternate
25" as "C<25*>".
=item * eastern_epact($year)
In the Julian comput, the epact is the age of the ecclesiastical Moon
on 22nd March. The C<eastern> part of the subroutine name accounts for
the fact that Gregorian and Julian calendars do not use the same
formula.
The epact is a 0..29 number. The "0" value is shown as "*" in some
sources. This subroutine does not convert "0" to "*", the result is
always a pure number. There is no other special case, for 25 as for
any other number.
The formula given by Reingold and Dershowitz is a "shifted epact" and
gives different results from the values printed in Lefort's and
Couderc's books. The module follows Couderc and Lefort.
=item * western_sunday_letter($year), eastern_sunday_letter($year)
On normal years (that is, excluding leap years), the Sunday letter is
determined by tagging 1st January with "A", 2nd January with "B", and
so on and looking at the first sunday of the year. The letter found at
this sunday if the sunday letter for the year.
The sunday letter governs all conversions from (mm, dd) to
day-of-week. For example, if the letter is "F", then 1st January, 12th
February, 2nd July and 1st October, among others, are tuesdays, while
6th January, 24th February, 14th July and 6th October are sundays.
On leap years, there are two sunday letters. The first one is
determined as above, the second one is determined by tagging 2nd
January, not 1st, with "A". The first sunday letter governs all
conversions from (mm, dd) to day-of-week for January and February
only, while the second sunday letter governs the conversions from (mm,
dd) to day-of-week for March and after.
So, if the sunday letters are "FE", 1st January and 12th February are
still tuesdays, but 2nd July and 1st October are wednesdays. At the
same time, 6th January and 24th February are still sundays, while 14th
July and 6th October are mondays.
C<western_sunday_letter> applies only to Gregorian years, while
C<eastern_sunday_letter> applies only to Julian years.
=item * western_sunday_number($year), eastern_sunday_number($year)
Letters (standalone or in pairs) are not convenient for numerical
calculations. So the I<xxx>C<_sunday_number> subroutine is used
instead of I<xxx>C<_sunday_letter>.
In case of leap years, the I<xxx>C<_sunday_number> subroutine gives
the numerical value for the second sunday letter, because Easter never
falls in January or February.
=item * easter($year)
Given a Gregorian year, this subroutine will return a DateTime object
for Western Easter Sunday in that year.
=item * western_easter($year)
Given a Gregorian year, this subroutine will return a DateTime object
for Western Easter Sunday in that year. But unlike the previous
subroutine, C<western_easter> cannot be imported. You must use its
fully qualified name:
lib/DateTime/Event/Easter.pm view on Meta::CPAN
If you build a list or a set of spans and if the C<from> or C<to> limits
coincide with the requested Easter event, the result may be different
from what you expect. For example, you ask for Easter sundays between
2017-04-16T21:43:00 and 2020-04-12T12:34:00.
The inclusive list or set will be:
2017-04-16T00:00:00 to 2017-04-16T23:59:59
2018-04-01T00:00:00 to 2018-04-01T23:59:59
2019-04-21T00:00:00 to 2019-04-21T23:59:59
2020-04-12T00:00:00 to 2020-04-12T23:59:59
and not:
2017-04-16T21:43:00 to 2017-04-16T23:59:59
2018-04-01T00:00:00 to 2018-04-01T23:59:59
2019-04-21T00:00:00 to 2019-04-21T23:59:59
2020-04-12T00:00:00 to 2020-04-12T12:34:00
The exclusive list or set will be:
2018-04-01T00:00:00 to 2018-04-01T23:59:59
2019-04-21T00:00:00 to 2019-04-21T23:59:59
and not:
2017-04-16T21:43:01 to 2017-04-16T23:59:59
2018-04-01T00:00:00 to 2018-04-01T23:59:59
2019-04-21T00:00:00 to 2019-04-21T23:59:59
2020-04-12T00:00:00 to 2020-04-12T12:35:59
Remarks and patches welcome.
Note for pedants: the hour C<21:43:01> should actually be
21 hours, 43 minutes, zero seconds and 1 nanosecond.
Likewise, all the times above ending with C<:59> include
999_999_999 nanoseconds.
=head2 Interaction of spans with timezones
It may happen that Palm sunday or Easter sunday coincide
with DST "spring forward" day (for Northern countries). I have not
checked what happens in this case for spans: a bit more than one day
for exactly 24 hours or exactly one day which gives 23 hours?
A similar question exists for DST "fall backward" day in the Southern
countries.
Also, since you can use a numeric C<day> offset beyond the Trinity
sunday, you can reach the Northern "fall backwards" and the Southern
"spring forward" days, where the same problem will happen in reverse.
=head1 THE SMALL PRINT
=head2 REFERENCES
=over 4
=item * L<https://github.com/houseabsolute/DateTime.pm/wiki> - The official wiki
of the DateTime project
=item * L<https://www.tondering.dk/claus/calendar.html> - Claus Tøndering's
calendar FAQ, especially the page L<https://www.tondering.dk/claus/cal/easter.php>.
=item * I<Calendrical Calculations> (Third or Fourth Edition) by Nachum Dershowitz and
Edward M. Reingold, Cambridge University Press, see
L<http://www.calendarists.com>
or L<https://www.cambridge.org/us/academic/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition?format=PB&isbn=9781107683167>,
ISBN 978-0-521-70238-6 for the third edition.
=item * I<La saga des calendriers>, by Jean Lefort, published by I<Belin> (I<Pour la Science>), ISBN 2-90929-003-5
See L<https://www.belin-editeur.com/la-saga-des-calendriers>
=item * I<Le Calendrier>, by Paul Couderc, published by I<Presses universitaires de France> (I<Que sais-je ?>), ISBN 2-13-036266-4
See L<https://catalogue.bnf.fr/ark:/12148/cb329699661>.
=back
=head2 SUPPORT
Support for this module, and for all DateTime modules will be given
through the DateTime mailing list - datetime@perl.org.
See L<https://lists.perl.org/list/datetime.html>.
Bugs should be reported through rt.cpan.org. See
L<https://rt.cpan.org/Public/Dist/Display.html?Name=DateTime-Event-Easter>.
Or you can try to submit a pull request to
L<https://github.com/jforget/DateTime-Event-Easter>.
=head2 AUTHOR
Rick Measham <rickm@cpan.org>
Co-maintainer Jean Forget <jforget@cpan.org>
=head2 CREDITS
Much help from the DateTime mailing list, especially from:
B<Eugene van der Pijll> - who pointed out flaws causing errors on
gregorian years with no eastern easter (like 35000) and who came up with
a patch to make the module accept any calendar's DateTime object
B<Dave Rolsky> - who picked nits, designed DateTime itself and leads the project
B<Martin Hasch> - who pointed out the posibility of memory leak with an early beta
B<Joe Orost> and B<Andreas König> - for RT tickets about the POD documentation
B<Frank Wiegand> and B<Slaven Rezić> - for patches fixing the POD problems
B<Tom Wyant> - for a constructive dialog about the relations between
L<DateTime::Calendar::Julian> (versions 0.101 and 0.102) and
L<DateTime::Event::Easter> (versions 1.08 and 1.09)
B<Andreas König> (again) - for a message a long time ago (December
2010) in which he told me about his web site
L<http://analysis.cpantesters.org/> which was very useful nine years
later for debugging DT::E::Easter version 1.08.
=head2 COPYRIGHT
© Copyright 2003, 2004, 2015, 2019 Rick Measham and Jean Forget. All
rights reserved. This program is free software; you can redistribute
it and/or modify it under the same terms as Perl itself: GNU
Public License version 1 or later and Perl Artistic License.
The full text of the license can be found in the F<LICENSE> file
included with this module or at
L<https://dev.perl.org/licenses/artistic.html>
and L<https://www.gnu.org/licenses/gpl-1.0.html>.
Here is the summary of GPL:
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 1, or (at your option)
any later version.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, see <https://www.gnu.org/licenses/> or
write to the Free Software Foundation, Inc., L<https://fsf.org>.
=head2 SEE ALSO
L<DateTime>, L<DateTime::Calendar::Julian>, perl(1)
L<https://metacpan.org/search?q=easter> which gives L<Date::Easter>, L<Date::Calc> and L<Date::Pcalc>
L<https://github.com/houseabsolute/DateTime.pm/wiki>
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