Running this code in latest 0.8 and 0.10 versions of node:

> date = new Date(2013, 0, 1)
Tue Jan 01 2013 00:00:00 GMT+0200 (EET)
> date.setUTCMonth(5)
1341180000000
> date
Mon Jul 02 2012 01:00:00 GMT+0300 (EEST)

The year changed from 2013 to 2012 after .setUTCMonth was called. Why did that happen?

The year changed from 2013 to 2012 after .Setutcmonth was called.

No, it did not.

Your local time zone is GMT+0200 – which means, your date that you see as Tue Jan 01 2013 00:00:00 GMT+0200 is really two hours earlier in GMT, so it’s December 31st 2012, 22:00:00.

And changing the month of that date to 5 makes your modified date end up being in July 2012 accordingly.