TimezoneJS should be a little bit more robust when test frameworks wrap the Date constructor.

No longer failing to detect dates when test frameworks overwrite the Date constructor (poorly)

Specifically, SinonJS overwrites the Date constructor with a function that produces date objects
whose constructor property is the original Date constructor, while Date currently refers to the Sinon wrapper.

That hullabaloo makes a.constructor !== Date end up true when it should be false.