Nodejs Utililty Methods Year Calculate

window = this;
Date.prototype.getYear = function() {
  return new Date().getFullYear();
};
function Error(message, description) {
  const e = new Error(message);
  e.description = description;
  return e;

var weekdays = exports.weekdays = ["sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"];
Date.prototype.increaseYear = function(num) {
  this.setUTCFullYear(this.getUTCFullYear() + num);
Date.prototype.increaseMonth = function(num) {
  var str = this.getUTCFullYear()+"-"+(this.getUTCMonth()+num + 1)+"-"+
        this.getUTCDate()+"T"+this.getUTCHours()+":"+this.getUTCMinutes()+":"+
        this.getUTCSeconds()+"."+this.getUTCMilliseconds()+"Z";
  var date = new Date(str);
...

Date.prototype.isThisYear = function() {
    return new Date().getYear() == this.getYear();
};

Date.prototype.subYears = function(value) {
  var month = this.getMonth();
  this.setFullYear(this.getFullYear() - value);
  if (month < this.getMonth()) {
    this.setDate(0);
  return this;
};

Date.prototype.toYearEnd = function () {
    var date = new Date(this);
    date.setMonth(12);
    date.setDate(0);
    return date;
};

Date.prototype.toYearStart = function () {
    var date = new Date(this);
    date.setMonth(0);
    date.setDate(1);
    return date;
};

Date.prototype.weekOfYear = function () {
    var year = this.getFullYear();
    var d = new Date(this);
    d.setHours(0, 0, 0, 0);
    var yearStart = new Date(year, 0, 1);
    d.setDate(d.getDate() + yearStart.getDay());
    var deltaDay = (d.getTime() - yearStart.getTime()) / 86400000 + 1;
    var weekNo = Math.ceil(deltaDay / 7);
    return [year, weekNo];
...

Date.prototype.yearStartDay = function(){
  return new Date(this.getFullYear(),0,1).getDay();