/**
 * Created by laixi on 2016/1/7.
 * Date對象擴展
 */
(function() {

    // 求當前日期與傳入的日期相隔多少天
    if (typeof Date.prototype.getDateInterval != "function") {
        Date.prototype.getDateInterval = function(date) {
            var d = new Date(date);
            if (d == "Invalid Date") {
                throw "Invalid Date";
            }else {
                // Math.abs 絕對值
                return Math.abs(this*1-d*1)/60/60/1000/24;
            }
        }
    }

    // 求當前日期所在月的第一天
    if (typeof Date.prototype.getFirstDateInMonth != "function") {
        Date.prototype.getFirstDateInMonth = function() {
            return new Date(this.getFullYear(), this.getMonth(), 1);
        }
    }

    // 求當前日期所在月的最後一天
    if (typeof Date.prototype.getLastDateInMonth != "function") {
        Date.prototype.getLastDateInMonth = function() {
            return new Date(this.getFullYear(), this.getMonth()+1, 0);
        }
    }

    // 求當前日期所在季度的第一天
    if (typeof Date.prototype.getFirstDateInQuarter != "function") {
        Date.prototype.getFirstDateInQuarter = function() {
            return new Date(this.getFullYear(), Math.floor(this.getMonth()/3)*3, 1);
        }
    }

    // 判斷是否為閏年
    if (typeof Date.prototype.isLeapYear != "function") {
        Date.prototype.isLeapYear = function() {
            return new Date(this.getFullYear(), 2, 0).getDate() == 29;
        }
    }

    // 求某年某月的天數
    if (typeof Date.daysInMonth != "function") {
        Date.daysInMonth = function(year, month) {
            var d = new Date();
            d.setFullYear(year, (month == 12) ? 1 : month, 0);
            return d.getDate();
        }
    }

}());

參考

http://www.cnblogs.com/rubylouvre/archive/2010/09/16/1827784.html