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Calculating Easter Sunday

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23 Nov 2001 14  
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.

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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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.

A list of licenses authors might use can be found here