Date of easter

chapters/dates_and_times/date-of-easter.textile

@@ -0,0 +1,50 @@
+---
+layout: recipe
+title: Calculate the date of Easter Sunday
+chapter: Dates and Times
+---
+
+h2. Problem
+
+You need to find the month and day of the Easter Sunday for given year.
+
+h2. Solution
+
+The following function returns array with two elements: month (1-12) and day of the Easter Sunday. If no arguments are given
+result is for the current year.
+This is an implementation of "Anonymous Gregorian algorithm":http://en.wikipedia.org/wiki/Computus#Anonymous_Gregorian_algorithm in CoffeeScript
+
+{% highlight coffeescript %}
+
+gregorianEaster = (year = (new Date).getFullYear()) ->
+  a = year % 19
+  b = ~~(year / 100)
+  c = year % 100
+  d = ~~(b / 4)
+  e = b % 4
+  f = ~~((b + 8) / 25)
+  g = ~~((b - f + 1) / 3)
+  h = (19 * a + b - d - g + 15) % 30
+  i = ~~(c / 4)
+  k = c % 4
+  l = (32 + 2 * e + 2 * i - h - k) % 7
+  m = ~~((a + 11 * h + 22 * l) / 451)
+  n = h + l - 7 * m + 114
+  month = ~~(n / 31)
+  day = (n % 31) + 1
+  [month, day]
+
+{% endhighlight %}
+
+h2. Discussion
+
+NB! Javascript numbers months from 0 to 11 so .getMonth() for date in March will return 2, this function will return 3.
+You can modify the function if you want this to be consistent.
+
+{% highlight coffeescript %}
+
+gregorianEaster()    # => [4, 24] (April 24th in 2011)
+gregorianEaster 1972 # => [4, 2]
+
+{% endhighlight %}
+

wanted-recipes.textile

@@ -42,7 +42,6 @@ evens.every even
 h2. Dates and Times
 
 * Calculating the phase of the moon
-* Holidays: Calculating Easter
 * Holidays: Calculating US Thanksgiving
 * Holidays: Calculating CA Thanksgiving
 * Number of days between two dates