Easter Date Calculation. Adapted from BASIC code found on Ronald W. Mallen's Easter Dating Method web page and written by Greg Mallen. To view the HTML and JavaScript source, see the View menu.

Tested successfully against:

Day Month Year
Enter Year (1583 to 4099)

Here's the original code for comparison with the main JavaScript function easterDateCalc():
  Sub EasterDate (d, m, y)

 ' EASTER DATE CALCULATION FOR YEARS 1583 TO 4099

 ' y is a 4 digit year 1583 to 4099
 ' d returns the day of the month of Easter
 ' m returns the month of Easter

 ' Easter Sunday is the Sunday following the Paschal Full Moon (PFM) date 
 ' for the year

 ' This algorithm derives values by iterative and inter-dependent processes, 
 ' so ... DO NOT MODIFY THE ORDER OF CALCULATIONS!

 ' Boolean expressions are used in calculations: -1 if true, 0 if false
 ' \ means integer division (so, 1995 \ 100 = 19)

   Dim a, b, c              ' intermediate results, all integers
   Dim tA, tB, tC, tD, tE   ' table A - E results, all integers

 ' y must be 1583 to 4099
    a = y \ 100
    b = y Mod 19

 ' calculate PFM date
    c = (a - 15) \ 2 + (a > 26) + (a > 38) + 202 - 11 * b
    c = c + ((a = 21) Or (a = 24) Or (a = 25) Or (a = 33) Or (a = 36) Or (a = 37))
    c = c Mod 30
    tA = c + (c = 29) + (c = 28 And b > 10) + 21   ' table A result (21=M21 to 49=A18)

 ' find the next Sunday
    tB = (tA - 19) Mod 7                   ' table B result
    c = (40 - a) Mod 4
    tC = c - (c > 1) - (c = 3)             ' table C result
    c = y Mod 100
    tD = (c + c \ 4) Mod 7                 ' table D result
    tE = ((20 - tB - tC - tD) Mod 7) + 1   ' table E result

 ' return the date
    d = tA + tE
    If d > 31 Then
        d = d - 31
        m = 4
    Else
        m = 3
    End If

 End Sub