Javascript Dates

Published March 19, 2012 · 1 Minute Read · ∞ Permalink

I’ve just spent half an hour wrestling with this, and for the benefit of those to come…

d = new Date(2011, 12, 08)

Will produce a new datetime object, right? No biggie, if you look inspect it, it displays like this:

"Thu Dec 08 2011 11:33:05 GMT-0600 (CST)"

But lo, the beast lurks. calling getFullYear and getDate work fine, and return 2011 and 8, respectively. But getMonth is a bit different. See, the Date object thinks of January as month 0, February as month 1, etc. So converting from one date to another (to get the first of the month at midnight, for example) would be done like this in Python:

>>> d = datetime.now()
>>> month = datetime(d.year, d.month, 1)

But has to be done like this in Javascript:

d = new Date() month = new Date(d.getYear(), d.getMonth() - 1, 1)

Yes, I know that methods like setDate exist, but constructing a new object with only what I need seems more efficient.

By the way, here’s the way that MongoDB deals with this in their ISODate:

var year = parseInt(res[1], 10) || 1970);
var month = (parseInt(res[1], 10) || 1) - 1;
var date = parseInt(res[3], 10) || 0;