I am working on a contract right now where I am having to do some manipulation of dates in a calendar. This particular calendar stores dates in Julian Day Number (JDN) format. This by itself is not a serious issue as PHP has simple functions to handle these conversions. However I have a situation where I have a Gregorian date in the DOM, and I need to pass the Julian date to a JavaScript function.

After a bit of research, and a few algorithms online that didn’t do the trick, I used a formula from a site called Astronomy Answers that seems to have done the trick.

Below is a piece of code I have written to convert a Gregorian date to a Julian Day Number, as well as a sample of using it.

Date.prototype.ToJulian = function() {
var d = this.getDate();
var y = this.getFullYear();
var m = this.getMonth() + 1;

return Math.floor((1461 * (y + 4800 + (m - 14) / 12)) / 4 + (367 * (m - 2 - 12 * ((m - 14) / 12))) / 12 - (3 * ((y + 4900 + (m - 14) / 12) / 100)) / 4 + d - 32075);
};

var testDate = "12/9/2009";
var parsedDate = new Date(Date.parse(testDate));
var julianDate = parsedDate.ToJulian();

alert(julianDate);

Let me know if you run into any issues with this calculation, or find a better way to do it. Also let me know if you can get the reverse to work (going from Julian to Gregorian), as this is giving me some issues!