UPDATE 2014-01-15: The current version of this Mathcad Prime 3.0 worksheet (along with a .pdf copy for users of older versions of Mathcad) can be found in the Puzzles and Games section. This version in the main documents section will not be updated further.--FEL
Stepping away from civil engineering for a bit…
This worksheet calculates the date for Easter Sunday using a modified version of the so-called "Meeus/Jones/Butcher" Gregorian Easter Algorithm that was presented by Ian Stewart in the March 2001 edition of Scientific American. The Gregorian Calendar was introduced in October 1582, so this calculation is only valid starting in 1583. Tidal friction is gradually slowing the Earth's rotation so at some point in the distant future this algorithm will require revision. We won't be around to do it.