Date.prototype javascript

Date.prototype.Format = function (sFormat) {
    if (typeof sFormat !== "string") 
        return this.getFullYear().toString() + 
               (this.getMonth() + 1).toString().PadLeft("0", 2) +
               this.getDate().toString().PadLeft("0", 2);
    var that = this;
    return sFormat.replace(/dd?d?d?|MM?M?M?|yy?y?y?|hh?|HH?|mm?|ss?/g,
    function (sFormat) {
        switch (sFormat) {
        case "yyyy": return that.getFullYear().toString();
        case "yy": return that.getFullYear().toString().substring(2, 4);
        case "MM": return (that.getMonth() + 1).toString().PadLeft(2, "0");
        case "dd": return that.getDate().toString().PadLeft(2, "0");
        case "hh": return (that.getHours() <= 12 ? that.getHours() : 
                   (that.getHours() - 12)).toString().PadLeft(2, "0");
        case "HH": return that.getHours().toString().PadLeft(2, "0");
        case "mm": return that.getMinutes().toString().PadLeft(2, "0");
        case "ss": return that.getSeconds().toString().PadLeft(2, "0");
        case "M": return (that.getMonth() + 1).toString();
        case "d": return that.getDate().toString();
        case "h": return (that.getHours() <= 12 ? that.getHours() : 
                  (that.getHours() - 12)).toString();
        case "H": return that.getHours().toString();
        case "m": return tht.getMinutes().toString();
        case "s": return that.getSeconds().toString();
        }
    });
};