Rebol[ file: %easter.r title: "dates of easter" date: 28-Feb-2005 purpose: "find the date Easter will fall on for a particular year" library: [ level: 'beginner platform: 'all type: 'tool domain: 'text tested-under: [xp solaris] support: none license: none see-also: none ] ] easter?: func [ {given a year returns the date of easter. lifted from "Astronomical Formulae for Caculators" pp 31 by Jean Meeus 1979 algorithm attributed to a 1876 publication note: I am adding a + 1 to the day to account for an appearent rounding error } x[integer!] "(Gregorian) year" /local a b c d e f g h i k l m n p ][ a: x // 19 b: to integer! (x / 100) c: x // 100 d: to integer! (b / 4) e: b // 4 f: to integer! (b + 8 / 25) g: to integer! (b - f + 1 / 3) h: 19 * a + b - d - g + 15 // 30 i: to integer! (c / 4) k: c // 4 l: 32 + e + e + i + i - h - k // 7 m: to integer! (11 * h + a + (22 * l) / 451) n: to integer! (h + l - (7 * m) + 114 / 31) p: h + l - (7 * m) + 114 // 31 to date! reduce[x n p + 1] ]