Moment.js - a javascript date library
Date manupulation is still one of the most complex problems while developing and it's a really nerve wracking work when developing in javascript. There is a very lightweight and nice framework for that called Moment.js. They offer functions to parse, validate, manipulate and display dates which made my live a lot easier.
For a while now they also offer a framework for the work with timezones called Moment Timezone to parse and display dates in any timezone.