We first get the timestamp for the 1st day of the year - number of seconds elapsed since midnight Coordinated Universal Time (UTC) of January 1, 1970, not counting leap seconds. This is done using the following piece of code
var timestmp = new Date().setFullYear(new Date().getFullYear(), 0, 1);
We then divide this timestamp by the total number of seconds in a day – 86400000. This gives us the number of days on Jan 1st of the current year, since the Unix epoch at January 1st 1970 midnight UTC
var yearFirstDay = Math.floor(timestmp / 86400000);
The next line calculates number of days since the Unix epoch at January 1st, 1970 midnight UTC till the current date, that is today.
var today = Math.ceil((new Date().getTime()) / 86400000);
At the end, we calculate the current day of the year by subtracting first day of the year – today.
var dayOfYear = today - yearFirstDay;
OUTPUT of the code if run today, is 217.