Algorithms

Richard MacCutchan wrote:

date of Easter for any year, based on the first full moon after March 21st.

Date.prototype.Easter   =   // Date of Easter Sunday for the year that the date object represents or for a given year
    function(optYear)   // Optionally can override year that the date is for
        // optYear can be a Date object or a numeric year
    {
        var year    = optYear
            ?   (   optYear.constructor == Date
                ?   optYear.getFullYear()
                :   ( optYear < 1900 ? optYear + 1900 : optYear )   // Assume number
                )
            :   this.getFullYear();
 
        // Algorithm optimised from 'Puzzles and Paradoxes' by T H O'Beirne
        // (Oxford University Press, London (c) 1965)
        var a   = year % 19;
        var b   = Math.floor(year / 100);
        var c   = year % 100;
        var d   = Math.floor(b / 4);
        var h   = (19 * a + b - d - Math.floor((8 * b + 13) / 25) + 15) % 30;
        var mu  = Math.floor((a + 11 * h) / 319) - h;
        var lambda  = (2 * (b - d * 4) + Math.floor(c / 4) * 6 - c + mu + 32) % 7 - mu;
        var month   = Math.floor((lambda + 90) / 25);
 
        return  new Date(year, month - 1, (lambda + month + 19) % 32);
    };  // Date.prototype.Easter