Question

In chrome console,

new Date('2012 01 01')
output: Sun Jan 01 2012 00:00:00 GMT-0600 (Central Standard Time)

new Date(2012, 01, 01)
output:
Wed Feb 01 2012 00:00:00 GMT-0600 (Central Standard Time)

I’m really curious as to why this happens.

Anyone care to shed some light?

Answer 1

I think you meant these:

new Date('2012 01 01');
new Date(2012, 01, 01); // Note the commas

And the reason is that the second example above uses the version of the Date constructor that accepts numbers, rather than a string, and month numbers in JavaScript start with 0 = January. So new Date(2012, 1, 1) (the leading 0 in your examples is technically an error, but most engines allow it and treat the number as octal) is February 1st 2012.

The first version above uses the Date constructor that takes a string and parses it, and when parsing a date string, month numbers typically start with 1 = January. Note that the string you’re asking Chrome to parse isn’t in any format defined by the spec, and other engines may not parse it. In fact, until ECMAScript 5, there was no defined date string format (the Date constructor that accepted a string was defined as accepting whatever toString output, but neither was actually specified). ECMAScript 5 added a simplified version of ISO-8601. But in the wild, every engine in the last decade has supported date strings in the form yyyy/mm/dd (but not necessarily the now-specified yyyy-mm-dd).