While I half agree with what you did, I think you're a step away from perfectness (see bellow) :)

I actually had the exact same problem and I was recently speaking to a friend from google and asked him to implement this as part of their search engine (i.e. just like the calculator, but for time zones) -- he didn't see the need for such a thing.

Here is my use case which doesn't work: conference deadlines. Please tell me what this:

Thursday, February 4, 2010, 11:59pm Eastern Standard Time (GMT −5:00)

translates to in my local time. Note that entering the date in your app does nothing, even if you split EST from the rest.