Handle Date.parse returning null or NaN

I'm running into the issue described in mde/timezone-js#100 too.

The problem is the isNaN check here:
https://github.com/mde/timezone-js/blob/f3efe06555c57be51cad708c7e19889264e89b83/src/date.js#L201

I've created a JSFiddle that demonstrates the issue:
http://jsfiddle.net/ttr2r/