Today I found a very weird bug in JavaScript – The date function. I never expected to see a bug in the standard JavaScript implementation. Here is my code. It does nothing more than adding 21 days on the reference date:

var referenceDateObject	= new Date(2011, 9, 10);
var deliveryDateObj 	= new Date(referenceDateObject.getTime() + 24 * 60 * 60 * 1000 * 21);
var deliveryDateMonth	= parseInt(deliveryDateObj.getUTCMonth());
var deliveryDateString	= deliveryDateObj.getUTCFullYear() + '-' + deliveryDateMonth + '-' +  deliveryDateObj.getUTCDate();
alert(deliveryDateString);

Given a reference date of September 10, 2011, I expect the date should be October 1, 2011. However, it ends up returning me September 31, which doesn’t exist!

I test that on Firefox and Google Chrome, both give the same problem. A quick fix will be moving the date calculation to the server side.

P.S. MySQL date function automatically convert 9/31 to 10/1, so the data on the server looks fine.

Update – That’s my fault. The Date object treats the month from 0-11 instead of 1-12. Here is a corrected version:

var referenceDateObject	= new Date(2011, 8, 10);   //That's September 10, 2011, not August 10.
var deliveryDateObj 	= new Date(referenceDateObject.getTime() + 24 * 60 * 60 * 1000 * 21);
var deliveryDateMonth	= parseInt(deliveryDateObj.getUTCMonth()) + 1;
var deliveryDateString	= deliveryDateObj.getUTCFullYear() + '-' + deliveryDateMonth + '-' +  deliveryDateObj.getUTCDate();
alert(deliveryDateString);

That will do the trick.

–Derrick