The calendar extension presents a series of functions to simplify converting between different calendar formats. The intermediary or standard it is based on is the Julian Day Count. The Julian Day Count is a count of days starting from January 1st, 4713 B.C. To convert between calendar systems, you must first convert to Julian Day Count, then to the calendar system of your choice. Julian Day Count is very different from the Julian Calendar! For more information on Julian Day Count, visit http://serendipity.magnet.ch/hermetic/cal_stud/jdn.htm. For more information on calendar systems visit http://genealogy.org/~scottlee/cal-overview.html. Excerpts from this page are included in these instructions, and are in quotes.
The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.
The following constants are available since PHP 4.3.0 :