TMZNS is a free, open-sourced timezone converter.
tmzns.com or as a Chrome Web Store app.This is now available at
Finding out the actual time difference between two places is hard. There are online converters that can do that for you, but their user experience sucks. I always wondered why someone doesn't fix this -- until I ultimately decided to try & fix it myself.
And boy, was I surprised how hard this problem is! If you think this is a simple algorithm, think again. Timezones and daylight saving times around the world are a mess. Fortunately, the great people at tzdata are publishing an open-source database of all the info needed. So I was able to focus on user experience.
This is also an exercise of using Google's Closure Library & Compiler, which is why the code looks structured and everything "just works" (although, as always, YMMV).