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Calculating Easter Sunday

, 23 Nov 2001
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How to find Easter Sunday and Ash Wednesday
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Introduction

Looking through old Pascal sources I found an algorithm to calculate Easter Sunday. I reinterpreted it to C++ and packed it into a small MFC dialog application.

The sample calculates Easter from 1970 to 2037. Responsible for the limitation is the function mktime, used to calculate the offset to ash. Basically GetEasterSunday is able to calculate Easter from 1583 (beginning of Gregorian calendar) to 2499 (end of correction values).

Ash Wednesday is 46 days before Easter Sunday.

void
CEasterDlg::GetEasterSunday( WORD wYear, WORD& wMonth, WORD& wDay )
{
    // calculate easter sunday
    // [in]  wYear  - 4-digit year to calculate (but not before 1583)
    // [out] wMonth - month of easter sunday
    // [out] wDay   - day of easter sunday

    WORD wCorrection = 0;   
    
    if( wYear < 1700 )      wCorrection = 4;
    else if( wYear < 1800 ) wCorrection = 5;
    else if( wYear < 1900 ) wCorrection = 6;
    else if( wYear < 2100 ) wCorrection = 0;
    else if( wYear < 2200 ) wCorrection = 1;
    else if( wYear < 2300 ) wCorrection = 2;
    else if( wYear < 2500 ) wCorrection = 3;

    wDay = (19 * (wYear % 19) + 24) % 30;        
    wDay = 22 + wDay + ((2 * (wYear % 4) + 4 * (wYear % 7) + 6 * wDay + 5 + wCorrection) % 7);
    
    // jump to next month
    if( wDay > 31 )
    {
        wMonth = 4;
        wDay -= 31;
    }
    else
    {
        wMonth = 3;
    }
}

Further reading:

http://aa.usno.navy.mil/data/docs/easter.html JavaScript that computes easter sunday.
http://www.hourworld.com/easter.htm More algorithms how to calculate.

Have fun.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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About the Author

Oskar Wieland
Web Developer
Germany Germany
No Biography provided

Comments and Discussions

 
GeneralThe algorithm is incorrect PinmemberSire40422-Mar-05 4:42 
GeneralThere are better algorithms. PinmemberWillzyba22-Nov-04 3:22 
GeneralIt does not take into account all the Christians PinmemberAlexandru Savescu27-Nov-01 12:39 
GeneralJust what I was looking for! PinmemberMatt Gullett26-Nov-01 9:09 
QuestionDead link? Pinmemberedx26-Nov-01 6:30 
Generalheheh PinmemberRui Lopes25-Nov-01 0:56 
Generalunusual :-) PinmemberNish [BusterBoy]24-Nov-01 20:09 

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