Will assigning 0 to the 3rd parameter of a JavaScript Date() object always create an end of month date?

Question

I'm working on a jQuery credit card expiration date validation script. Credit cards expire after the last day of the expiration month. For instance, if the card expires on 8/2013 then it's good through 8/31/2013.

In the past on the server side I've determined the last day of the month by adding 1 to the current month, then subtracting 1 day.

Today I noticed that when creating a new date, if 0 is applied to the 3rd parameter of the JavaScript Date() object, the resulting date will be the end-of-month day. But I've been unable to locate any online documentation to affirm this observation.

Here is some sample code.

var month = 10;
var year = 2013;
var expires = new Date(year, month, 0);
alert(expires);

And here is a jsFiddle example that I created.

This is a bit confusing, because I thought in JavaScript months were zero based. I've tested this in Chrome, Firefox, IE, and Safari, and the behavior appears consistent. The returned date consistently displays the last day of the month. This looks like a lucky find, but I'd really like to understand what is happening here.

Am I safe to run with this approach to assigning an end of month date, and if so is there some online documentation that I can point to which affirms this? Thanks.

Answer 1

Months are zero-based. That creates an end-of-month date in the previous month. Month 10 is November, so creating a date with day 0 in November gives you the end of October (month 9).

That is, day 0 in November means "the day before 1 November", which is the last day of October. Day -1 in November would be 30 October.