Javascript Date Format prototype Extention

Date.prototype.Extention.js

//Date.prototype
Date.prototype.Format = function( str ){
    var date = this;
    return str.replace(/%(Y|y|n|m|d|j|a|A|g|G|h|H|i|s|DD|ll|D|l|%)/g, function( word ){
        switch( word ){
            case "%%": return '%'; break;
            case "%Y": return date.getFullYear(); break;
            case "%y": return String(date.getFullYear()).substring(2,4); break;
            case "%n": return Number(date.getMonth())+1; break;
            case "%m": return (date.getMonth() < 9 ? "0" : "" )+(date.getMonth()+1); break;
            case "%d": return (date.getDate() < 10 ? "0" : "" )+date.getDate(); break;
            case "%j": return date.getDate(); break;
            case "%a": return (date.getHours() < 12 ? "am":"pm"); break;
            case "%A": return (date.getHours() < 12 ? "오전":"오후"); break;
            case "%g": return (date.getHours() < 12 ? date.getHours() : Number(date.getHours())-12); break;
            case "%G": return date.getHours(); break;
            case "%h": var tmp = (date.getHours() < 12 ? date.getHours() : Number(date.getHours())-12); return (tmp < 10 ? "0":"" )+tmp; break;
            case "%H": return (date.getHours() < 10 ? "0" : "" )+date.getHours(); break;
            case "%i": return (date.getMinutes() < 10 ? "0" : "" )+date.getMinutes(); break;
            case "%s": return (date.getSeconds() < 10 ? "0" : "" )+date.getSeconds(); break;
            case "%D": return ["Sun","Mon","Tue","Wed","Thu","Fri","Sat"][date.getDay()]; break;
            case "%l": return ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"][date.getDay()]; break;
            case "%DD": return ["일","월","화","수","목","금","토"][date.getDay()]; break;
            case "%ll": return ["일요일","월요일","화요일","수요일","목요일","금요일","토요일"][date.getDay()]; break;
            default : return word;
        }
    });
};

Date.prototype.Day = function( num ) {
    if( !num ) { return this; };
    return new Date( this.getTime() + 86400000 * num );
}

Date.lastDate =  function (y,m) {
    var count = 31;
    if (m == 2) { count = ((y % 4 == 0 && y % 100 != 0) || y % 400 == 0) ? 29 : 28; }
    else if (m == 4 || m == 6 || m == 9 || m == 11) { count = 30; }
    return count;
};

Date.prototype.Month = function( num ) {
    if( !num ) { return this; };
    var D = this;
    var y = D.getFullYear();
    var m = D.getMonth()+1;
    var d = D.getDate();
    var month = m + num;
    var abs = Math.abs( month );
    //+ or -
    if( month > 0 ) {
        y = y + ~~( ( month - 1 ) / 12 );
        m = abs % 12 == 0 ? 12 : abs % 12;
    }
    else {
        y = y + ~~( month / 12 ) - 1;
        m = 12 - abs % 12;
    }
    //28~31day
    var lastDate = Date.lastDate(y,m);
    if( d > lastDate ) { d = lastDate }
    return new Date( y + '/' + m + '/' + d );
};

Date.prototype.moveDay = function( num ) {
    if( !num || num == 0 ) { return this; };
    return new Date( this.getTime() + 86400000 * num );
}
Date.prototype.moveWeek = function( num ) {
    if( !num ) { return this; };
    return this.Day( 7 * num );
};
Date.prototype.nextWeek = function(num) { return this.moveWeek((num||1)*1); }
Date.prototype.lastWeek = function(num) { return this.moveWeek((num||1)*-1); }
Date.prototype.moveMonth = function( num ) {
    if( !num ) { return this; };
    var D = this;
    var y = D.getFullYear();
    var m = D.getMonth()+1;
    var d = D.getDate();
    var month = m + num;
    var abs = Math.abs( month );
    //+ or -
    if( month > 0 ) {
        y = y + ~~( ( month - 1 ) / 12 );
        m = abs % 12 == 0 ? 12 : abs % 12;
    }
    else {
        y = y + ~~( month / 12 ) - 1;
        m = 12 - abs % 12;
    }
    //28~31day
    var lastDate = Date.lastDate(y,m);
    if( d > lastDate ) { d = lastDate }
    var ret = new Date( y + '/' + m + '/' + d );
    ret.setHours( D.getHours() );
    ret.setMinutes( D.getMinutes() );
    ret.setSeconds( D.getSeconds() );
    return ret;
};
Date.prototype.nextMonth = function(num) { return this.moveMonth((num||1)*1); }
Date.prototype.lastMonth = function(num) { return this.moveMonth((num||1)*-1); }
Date.prototype.moveYear = function( num ) {
    if( !num ) { return this; };
    var D = this;
    var y = D.getFullYear();
    var m = D.getMonth()+1;
    var d = D.getDate();
    y = y + num;
    y = y < 1970 ? 1970 : y; //after 1970
    //28~31day
    var lastDate = Date.lastDate(y,m);
    if( d > lastDate ) { d = lastDate }
    var ret = new Date( y + '/' + m + '/' + d );
    ret.setHours( D.getHours() );
    ret.setMinutes( D.getMinutes() );
    ret.setSeconds( D.getSeconds() );
    return ret;
};
Date.prototype.nextYear = function(num) { return this.moveYear((num||1)*1); }
Date.prototype.lastYear = function(num) { return this.moveYear((num||1)*-1); }
Date.prototype.UTC = function() {
    if( Date.UTC ) {
        return new Date(Date.UTC(this.getFullYear(),this.getMonth(),this.getDate(),this.getHours(),this.getMinutes(),this.getSeconds()));
    }
    return this;
}