Javascript powerful date function code sharing_javascript skills

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var date = function( a, s ) {
    var d = s ? new Date( s ) : new Date(), f = d.getTime();
    return ( '' a ).replace( /a|A|d|D|F|g|G|h|H|i|I|j|l|L|m|M|n|s|S|t|T|U|w|y|Y|z|Z/g, function ( a ) {
        switch ( a ) {
                case 'a' : return d.getHours() > 11 ? 'pm' : 'am';
                case 'A' : return d.getHours() > 11 ? 'PM' : 'AM';
                case 'd' : return ( '0' d.getDate() ).slice(-2);
                case 'D' : return [ 'Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat' ][ d.getDay() ];
                case 'F' : return [ 'January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December' ][ d.getMonth() ];
                case 'g' : return ( s = ( d.getHours() || 12 ) ) > 12 ? s - 12 : s;
                case 'G' : return d.getHours();
                case 'h' : return ( '0' ( ( s = d.getHours() || 12 ) > 12 ? s - 12 : s ) ).slice(-2);
                case 'H' : return ( '0' d.getHours() ).slice(-2);
                case 'i' : return ( '0' d.getMinutes() ).slice(-2);
                case 'I' : return (function(){ d.setDate(1); d.setMonth(0); s = [ d.getTimezoneOffset() ]; d.setMonth(6); s[1] = d.getTimezoneOffset(); d.setTime( f ); return s[0] == s[1] ? 0 : 1; })();
                case 'j' : return d.getDate();
                case 'l' : return [ 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday' ][ d.getDay() ];
                case 'L' : return ( s = d.getFullYear() ) % 4 == 0 && ( s % 100 != 0 || s % 400 == 0 ) ? 1 : 0;
                case 'm' : return ( '0' ( d.getMonth() 1 ) ).slice(-2);
                case 'M' : return [ 'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec' ][ d.getMonth() ];
                case 'n' : return d.getMonth() 1;
                case 's' : return ( '0' d.getSeconds() ).slice(-2);
                case 'S' : return [ 'th', 'st', 'nd', 'rd' ][ ( s = d.getDate() ) < 4 ? s : 0 ];
                case 't' : return (function(){ d.setDate(32); s = 32 - d.getDate(); d.setTime( f ); return s; })();
                case 'T' : return 'UTC';
                case 'U' : return ( '' f ).slice( 0, -3 );
                case 'w' : return d.getDay();
                case 'y' : return ( '' d.getFullYear() ).slice(-2);
                case 'Y' : return d.getFullYear();
                case 'z' : return (function(){ d.setMonth(0); return d.setTime( f - d.setDate(1) ) / 86400000; })();
                default : return -d.getTimezoneOffset() * 60;
            };
    } );
};