Just another option (which I wrote - full disclosure):
DP_DateExtensions Library
Supports date/time formatting, date math (add/subtract date parts), date compare, date parsing, etc. It's liberally open sourced.
I really like Date.js but find some of the syntax cumbersome. Most of the functions in mine are based (at least in part) on the rich set of fuctions in CFML - some people may find it a bit more comfortable.
No reason to consider it if you're already using a framework (they're all capable), but if you just need to quickly add date manipulation to a project give it a chance.