(PHP 3 >= 3.0.9, PHP 4, PHP 5)easter_date -- Get Unix timestamp for midnight on Easter of a given year
Returns the Unix timestamp corresponding to midnight on Easter of the given year.
Since PHP 4.3.0, the
parameter is optional and defaults to the current year according to the
localtime if omitted.
This function will generate a warning if the year is outside of the range for Unix timestamps (i.e. before 1970 or after 2037).
The date of Easter Day was defined by the Council of Nicaea in AD325 as the Sunday after the first full moon which falls on or after the Spring Equinox. The Equinox is assumed to always fall on 21st March, so the calculation reduces to determining the date of the full moon and the date of the following Sunday. The algorithm used here was introduced around the year 532 by Dionysius Exiguus. Under the Julian Calendar (for years before 1753) a simple 19-year cycle is used to track the phases of the Moon. Under the Gregorian Calendar (for years after 1753 - devised by Clavius and Lilius, and introduced by Pope Gregory XIII in October 1582, and into Britain and its then colonies in September 1752) two correction factors are added to make the cycle more accurate.
(The code is based on a C program by Simon Kershaw, <webmaster at ely.anglican dot org>)
See easter_days() for calculating Easter before 1970 or after 2037.