function pad(number, length) {

    var str = '' + number;
    while (str.length < length) {
        str = '0' + str;
    }

    return str;

}

//yyyyMMdd()，format a Date Object to the format of "yyyyMMdd";
Date.prototype.yyyyMMdd = function () {
    var yyyy = this.getFullYear().toString();
    var MM = pad(this.getMonth() + 1, 2);
    var dd = pad(this.getDate(), 2);

    return yyyy + MM + dd;
};

//yyyyMMddHHmmss()，format a Date Object to the format of "yyyyMMddHHmmss";
Date.prototype.yyyyMMddHHmmss = function () {
    var yyyy = this.getFullYear().toString();
    var MM = pad(this.getMonth() + 1, 2);
    var dd = pad(this.getDate(), 2);
    var HH = pad(this.getHours(), 2);
    var mm = pad(this.getMinutes(), 2);
    var ss = pad(this.getSeconds(), 2);

    return yyyy + MM + dd + HH + mm + ss;
};

//parseDateWithDate, 格式yyyyMMdd；如果不符合日期格式，就返回undefined
function parseDateWithDate(dateStr) {
    // 正则表达式的match数组
    var dateMatch;
    // validate year as 4 digits, month as 01-12, day as 01-31
    if ((dateMatch = dateStr.match(/^(\d{4})(0[1-9]|1[0-2])(0[1-9]|[12]\d|3[01])$/))) {
        // make a date
        return new Date(+dateMatch[1], +dateMatch[2] - 1, +dateMatch[3]);
    }
    return undefined;
}

//parseDateWithSecond, 格式yyyyMMddHHmmss；如果不符合日期格式，就返回undefined
function parseDateWithSecond(dateStr) {
    // 正则表达式的match数组
    var dateMatch;
    // validate year as 4 digits, month as 01-12, day as 01-31, hour as 00-23, minute as 00-59, and second as 00-59
    if ((dateMatch = dateStr.match(/^(\d{4})(0[1-9]|1[0-2])(0[1-9]|[12]\d|3[01])([01]\d|2[0-3])([0-5]\d)([0-5]\d)$/))) {
        // make a date
        return new Date(+dateMatch[1], +dateMatch[2] - 1, +dateMatch[3], +dateMatch[4], +dateMatch[5], +dateMatch[6]);
    }
    return undefined;
}

var d1 = new Date(2013, 0, 1);
var d2 = new Date(2013, 0, 2);
d1 <  d2; // true
d1 <= d2; // true
d1 >  d2; // false
d1 >= d2; // false

var d1 = new Date(2013, 0, 1);
var d2 = new Date(2013, 0, 1);
/*
 * note: d1 == d2 returns false as described above
 */
d1.getTime() == d2.getTime(); // true
d1.valueOf() == d2.valueOf(); // true
Number(d1)   == Number(d2);   // true
+d1          == +d2;          // true