Every now and then, something really complex comes along and the answer is there on Google. A bit back now, I was asked to create a file for import into the organisations payroll system. The tricky thing was that this had to be an automated procedure that knew the date on which we would be paid.

The rule of thumb is fairly simple: We get paid on the 15th of the month. If this day happens to be a Saturday or Sunday - then it should be the preceding Friday. So sometimes, the 13th or 14th maybe. Bank Holidays? Most bank holidays avoid the 15th of the month without a problem - the exception is Easter. Easter is based on the lunisolar calendar. I had a quick Google, but alas, the Moon didn't have a web-service, so I was gonna have to do this the hard way!

Having a quick read up about the subject reveals a wealth of complications where, between them, the Gregorian calendar and Christianity just about mess everything up for everyone! Unthoughtful, I agree. But the general rule of thumb is:

Easter and the holidays that are related to it are moveable feasts, in that they do not fall on a fixed date in the Gregorian or Julian calendars (both of which follow the cycle of the sun and the seasons). Instead, the date for Easter is determined on a lunisolar calendar, as is the Hebrew calendar.

In Western Christianity, using the Gregorian calendar, Easter always falls on a Sunday between March 22 and April 25 inclusively.[36] The following day, Easter Monday, is a legal holiday in many countries with predominantly Christian traditions. In Eastern Christianity, which use the Julian calendar for religious dating, Easter also falls on a Sunday between March 22 and April 25 inclusive of the Julian calendar. In terms of the Gregorian calendar, due to the 13 day difference between the calendars between 1900 and 2099, these dates are between April 4 and May 8 inclusive.

I could have settled for programming the relevant dates for the next 40 years, but somehow, that didn't sit comfortably with me. The link I had back then that helped me work it out no longer exists, however, the basic premise is as follows.

To work out the date of Easter Sunday, there is some mathematical algorithms to it after all! (Praise be to science!)

These are the original comments, which are well written and I shan't be able to improve upon much:

First work out "epact": the approximate date of the Paschal moon. The number is the approximate number of days before April 19 that a full moon will occur. The number increases by 11 each year because 365 days is equal to 12 lunar cycles of 29.5 days plus 11 days. The 2 is an arbitrary start point based on the state of the moon when they worked out this algorithm is 1582.

"a" and "b" are corrections based on the current century.

The formula was based on the simpler one in use with the Julian calendar, so the first correction is just to make it work with the Gregorian calendar. It is essentially the "missing" leap days in the Gregorian calendar for 1700, 1800, etc, which is 15 for years 1900-2099.

The second correction improves the 19 years = 235 moons approximation.

Then the whole sum is reduced mod 30 to give us the age of the moon in days on April 5th (14 = full)

And a last correction is made in case either the epact is 0 (which would push the date of the moon later than the set limit of 18 April) or it is 1 AND we are in the second half of the 19 year cycle (because that means the previous correction was made already in this cycle but we don't want to repeat it) In either case we just bump it on 1 because e=2 cannot happen.

Genius! There was a wonderful example function written in a language which wasn't much use to me, so I converted into a VB Script function that could be called by my script. While the following is written for Classic ASP / VBScript, there is no reason why it can't be easily turned back into PHP, Java, or a VB or C#.NET function.

Function EasterMonday(iYear)
 
  Dim iMonth, iDay, iMoon, iEpact, iSunDay, iGold, iCent, iCorx, iCorz
 
  If IsNumeric (iYear) Then
    iYear = CInt(iYear)
 
    If (iYear >= 1583) And (iYear <= 8702) Then
      iGold = ((iYear Mod 19) + 1) 'the golden number of the year in the 19 year metonic cycle
      iCent = ((Int(iYear / 100)) + 1) 'calculate the century
      iCorx = (Int((3 * iCent) / 4) - 12) 'no. of years in which leap year was dropped in order to keep in step with the sun
      iCorz = (Int((8 * iCent + 5) / 25) - 5) 'special correction to syncronize easter with the moon's orbit
      iSunDay = (Int((5 * iYear) / 4) - iCorx - 10) 'find sunday
      iEpact = ((11 * iGold + 20 + iCorz - iCorx) Mod 30) 'set epact (specifies occurance of full moon

      If (iEpact < 0) Then
        iEpact = iEpact + 30
      End If
 
      If ((iEpact = 25) And (iGold > 11)) Or (iEpact = 24) Then
        iEpact = iEpact + 1
      End If
 
      iMoon = 44 - iEpact 'Find Full Moon

      If (iMoon < 21) Then
        iMoon = iMoon + 30
      End If
 
      iMoon = (iMoon + 7 - ((iSunDay + iMoon) Mod 7)) 'advance to sunday

      If (iMoon > 31) Then
        iMonth = 4
        iDay = (iMoon - 31)
      Else
        iMonth = 3
        iDay = iMoon
      End If
 
      EasterSunday = DateSerial(iYear, iMonth, iDay)
    Else
      EasterSunday = False
    End If
  Else
    EasterSunday = False
  End If
End Function

I'm always amazed by things like this. What initially seemed to me to be an overwhelming task, was solved by a small amount of code. What I thought was entirely based about the moon and how it felt like eclipsing every now and then, can actually be solved by mathematical equations!

I'd think about chalking this up as another victory for science over religion. However, I'm not entirely convinced that science and the stupidity of man didn't pay part in the confusion in the first place. So I'll just leave that potentially controversial debate for another time.