Extending Javascript's Date.prototype

extendDatePrototype.js

Date.prototype.getDayofYear = function(){
    var d= new Date(this.getFullYear(), 0, 0);
    return Math.floor((this-d)/8.64e+7);
}

Date.prototype.addDays = function (n) {
    var year = this.getFullYear();
    var month = this.getMonth();
    var date = this.getDate() + n;
    newDate = new Date(year, month, date);
    return newDate;
}

Date.prototype.occuredAfter = function (dateObj) {
    var diffMS = this.getTime() - dateObj.getTime();
    var occured = diffMS >= 0 ? true : false;
    return occured;
}

Date.prototype.getDaysSince = function (dateObj) {
    var oneDayMS = 1000 * 60 * 60 * 24;
    var diffMS = this.getTime() - dateObj.getTime();
    var diffDays = Math.round(diffMS/oneDayMS);
    return diffDays
}

Useful methods I've found/developed for extending Javascript's native Date object.