JavaScript calculate the day of the year (1 - 366)

Following OP's edit:

var now = new Date();
var start = new Date(now.getFullYear(), 0, 0);
var diff = now - start;
var oneDay = 1000 * 60 * 60 * 24;
var day = Math.floor(diff / oneDay);
console.log('Day of year: ' + day);

Edit: The code above will fail when now is a date in between march 26th and October 29th andnow's time is before 1AM (eg 00:59:59). This is due to the code not taking daylight savings time into account. You should compensate for this:

var now = new Date();
var start = new Date(now.getFullYear(), 0, 0);
var diff = (now - start) + ((start.getTimezoneOffset() - now.getTimezoneOffset()) * 60 * 1000);
var oneDay = 1000 * 60 * 60 * 24;
var day = Math.floor(diff / oneDay);
console.log('Day of year: ' + day);

I find it very interesting that no one considered using UTC since it is not subject to DST. Therefore, I propose the following:

function daysIntoYear(date){
    return (Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()) - Date.UTC(date.getFullYear(), 0, 0)) / 24 / 60 / 60 / 1000;
}

You can test it with the following:

[new Date(2016,0,1), new Date(2016,1,1), new Date(2016,2,1), new Date(2016,5,1), new Date(2016,11,31)]
    .forEach(d => 
        console.log(`${d.toLocaleDateString()} is ${daysIntoYear(d)} days into the year`));

Which outputs for the leap year 2016 (verified using http://www.epochconverter.com/days/2016):

1/1/2016 is 1 days into the year
2/1/2016 is 32 days into the year
3/1/2016 is 61 days into the year
6/1/2016 is 153 days into the year
12/31/2016 is 366 days into the year

Date.prototype.dayOfYear= function(){
    var j1= new Date(this);
    j1.setMonth(0, 0);
    return Math.round((this-j1)/8.64e7);
}

alert(new Date().dayOfYear())

This works across Daylight Savings Time changes in all countries (the "noon" one above doesn't work in Australia):

Date.prototype.isLeapYear = function() {
    var year = this.getFullYear();
    if((year & 3) != 0) return false;
    return ((year % 100) != 0 || (year % 400) == 0);
};

// Get Day of Year
Date.prototype.getDOY = function() {
    var dayCount = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334];
    var mn = this.getMonth();
    var dn = this.getDate();
    var dayOfYear = dayCount[mn] + dn;
    if(mn > 1 && this.isLeapYear()) dayOfYear++;
    return dayOfYear;
};