Date.isLeapYear = function (year) {
return (((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0));
};
Date.getDaysInMonth = function (year, month) {
return [31, (Date.isLeapYear(year) ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
};
Date.prototype.isLeapYear = function () {
return Date.isLeapYear(this.getFullYear());
};
Date.prototype.getDaysInMonth = function () {
return Date.getDaysInMonth(this.getFullYear(), this.getMonth());
};
Date.prototype.addMonths = function (value) {
var n = this.getDate();
this.setDate(1);
this.setMonth(this.getMonth() + value);
this.setDate(Math.min(n, this.getDaysInMonth()));
return this;
};