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Calculating Easter Sunday

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23 Nov 2001 95.2K   530   15   7
How to find Easter Sunday and Ash Wednesday

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.

C++
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

Have fun!

License

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Comments and Discussions

 
GeneralThe algorithm is incorrect Pin
Sire40422-Mar-05 4:42
Sire40422-Mar-05 4:42 
GeneralThere are better algorithms. Pin
Willzyba22-Nov-04 3:22
Willzyba22-Nov-04 3:22 
GeneralIt does not take into account all the Christians Pin
Alexandru Savescu27-Nov-01 12:39
Alexandru Savescu27-Nov-01 12:39 
GeneralJust what I was looking for! Pin
Matt Gullett26-Nov-01 9:09
Matt Gullett26-Nov-01 9:09 
Thanks!
QuestionDead link? Pin
edx26-Nov-01 6:30
edx26-Nov-01 6:30 
Generalheheh Pin
Rui Lopes25-Nov-01 0:56
Rui Lopes25-Nov-01 0:56 
Generalunusual :-) Pin
Nish Nishant24-Nov-01 20:09
sitebuilderNish Nishant24-Nov-01 20:09 

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