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Doc Bug #12766 easter_days breaks for year before 1753
Submitted: 2001-08-15 10:51 UTC Modified: 2002-06-10 03:54 UTC
From: martin at diers dot cc Assigned:
Status: Closed Package: Documentation problem
PHP Version: 4.0.6 OS: Linux 2.4.x
Private report: No CVE-ID: None
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 [2001-08-15 10:51 UTC] martin at diers dot cc 
The easter_days function returns bad data for years <= 1752. Here is a way to verify. Because Easter always falls on a Sunday, the following function should always return "Sunday":

    function EasterDOW($year) {
        $jdayc = easter_days($year);
        $jdmar21 = gregoriantojd(3, 21, $year);
	$jdeaster = $jdmar21 + $jdayc;
        return JDDayOfWeek($jdeaster, 1);
    }

The formula for calculating easter has been the same since 1582. The standerd Delambre Easter algorithm is able to generate the date of easter for any gregorian date after 1582. The following function may be used to duplicate the functionality of easter_days for any date since 1582:

    function easter_days2($year) {
    
        #First calculate the date of easter using
        #Delambre's algorithm.
        $a = $year % 19;
        $b = floor($year / 100);
        $c = $year % 100;
        $d = floor($b / 4);
        $e = $b % 4;
        $f = floor(($b + 8) / 25);
        $g = floor(($b - $f + 1) / 3);
        $h = (19 * $a + $b - $d - $g + 15) % 30;
        $i = floor($c / 4);
        $k = $c % 4;
        $l = (32 + 2 * $e + 2 * $i - $h - $k) % 7;
        $m = floor(($a + 11 * $h + 22 * $l) / 451);
        $n = ($h + $l - 7 * $m + 114);
        $month = floor($n / 31);
        $day = $n % 31 + 1;

        #Return the difference between the JulianDayCount
        #for easter and March 21'st of the same year,
        #in order to duplicate the functionality of the
        #easter_days function
        return GregorianToJD($month, $day, $year) - GregorianToJD(3,21,$year);
    }

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 [2001-08-15 14:25 UTC] martin at diers dot cc 
Based on the documentation for the easter_days command, this behaviour would appear to be deliberate. It seems that the easter calculation is changed over to the Julian formula for all dates prior to 1753, because that is the year that Britain and its colonies finally switched to the Gregorian calendar. However, that does not change the fact that since 1582, the rest of Europe was using the Gregorian calendar.

This being the case, it would make since to be able to calculate the date of Easter for the range 1582-1752 using either the Julian formula (which is simply based on cycles of the moon) or the Gregorian formula (which is far more complex). To implement this, an optional switch could be added to the easter_days function which defaults to the Julian formula for that date range, but allows the user to specify the Gregorian formula instead.
 [2002-04-09 18:53 UTC] jimw@php.net 
yes, the fallibility of this function is well-documented.

reclassifying as a feature request.
 [2002-05-12 00:00 UTC] php-bugs at lists dot php dot net 
No feedback was provided for this bug for over a month, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".
 [2002-05-12 03:18 UTC] mfischer@php.net 
Feature Request -> Status: Open.
 [2002-05-12 11:15 UTC] markonen@php.net 
This issue has been resolved in the CVS version (4.3.0-
dev). You can fetch a snapshot with the improved 
functionality from http://snaps.php.net/

An optional second parameter was added to easter_days().
It can take one of four values:

CAL_EASTER_DEFAULT for the current (and default) 
functionality, CAL_EASTER_ROMAN for continental european 
behaviour (changes the Julian > Gregorian cut off date from 
1752 to 1582) and CAL_EASTER_ALWAYS_JULIAN or 
CAL_EASTER_ALWAYS_GREGORIAN for forcing the calendar 
regardless of date.

I'm reclassifying this as a documentation problem in order 
to get the new feature documented.

Thank you for your bug report, and for helping us make PHP 
better.
 [2002-06-10 03:54 UTC] mfischer@php.net 
This bug has been fixed in CVS. You can grab a snapshot of the
CVS version at http://snaps.php.net/. In case this was a documentation 
problem, the fix will show up soon at http://www.php.net/manual/.
In case this was a PHP.net website problem, the change will show
up on the PHP.net site and on the mirror sites.
Thank you for the report, and for helping us make PHP better.
 
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