Easter ( y )

Given the year, Y, computes the Easter date for years 1583 to 4099

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Phil Caulkins aka PhilModJunk
Caulkins Consulting
Http::www.Don'tHaveASite

Sample input:

Easter ( 2012 )

Sample output:

4/8/2012

Function definition: (Copy & paste into FileMaker's Edit Custom Function window)

// Easter ( Y )
//
// Given the year, Y, computes the Easter date for years 1583 to 4099
// Created by Phil Caulkins using alogrithm from 
// http://christianity.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=christianity&cdn=religion&tm=166&f=00&tt=13&bt=1&bts=0&zu=http%3A//www.assa.org.au/edm.html
//
Let ( [ FirstDig = Left ( y ; 2 ) ;          // first 2 digits of year
          Remain19 = Mod ( y ; 19 ) ;    // remainder of year / 19
          temp = Div ( (FirstDig - 15) ; 2 ) + 202 - 11 * Remain19 ;
          
          //Note that for all years prior to 2100, you can just use Mod ( Temp ; 30 ) in place of this case function
          temp2 = Case ( ValueCount ( FilterValues ( List ( 21 ; 24 ; 25 ; 27 ; 28 ; 29 ; 30 ; 31 ; 32 ; 34 ; 35 ; 38 ) ; FirstDig ) ) ; Mod ( temp - 1 ; 30 );
                                    ValueCount ( FilterValues ( List ( 33 ; 36 ; 37 ; 39 ; 40 ) ;  FirstDig ) ) ; Mod ( temp - 2 ; 30 ) ;
                                     /* else */ Mod ( temp ; 30 ) 
                                   )  ;
          tA = temp2 + 21 ;
          tA2 = Case ( temp2 = 29 ; tA - 1 ;
                                temp2 = 28 and Remain19 > 10 ;  tA - 1;
                               /* else */ tA
                             );
           //Find the next Sunday 
            tB = Mod ( tA2 - 19 ; 7 ) ;
    
            tC = Mod ( 40 - FirstDig ; 4 ) ;
            tC2 = Case ( tC = 3 ; tC + 2 ;  //must be + 2 to match succeeding step that also increments by 1 in original algorithm
                                  tC > 1 ; tC + 1 ;
                                 /* else */ tc 
                               ) ;
           temp3 = Mod ( y ; 100 ) ;
          tD = Mod ( temp3 + Div ( temp3 ; 4 ) ; 7 ) ;

           tE = Mod (20 - tB - tC2 - tD ; 7 ) + 1 ;
           d = tA2 + tE // day of month but not yet adjusted for values greater than 31
          ] ;

           //compute the date from these temp values
          If ( d > 31 ; Date ( 4 ; d-31 ; y ) ; Date ( 3 ; d ; y ) )
      ) // let

Needed an algorithm for computing the Western date of Easter Sunday for the Gregorian Calendar.

Found an algorithm at

http://christianity.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=christianity&cdn=religion&tm=166&f=00&tt=13&bt=1&bts=0&zu=http%3A//www.assa.org.au/edm.html

and adapted it to implement it in a FileMaker Custom function.

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Easter ( y )

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