Date display behavior should be well-defined and tested

[StephenChan]

It'd be nice to settle on answers for the following:

• Do we display dates in the server's local time? UTC time? In the user's timezone?
• What's the best way to detect the user's timezone, if we go that route?
• Should dates be formatted only one way, or should we detect user locales and match those?

And then the behavior should be well-defined in the following: source creation dates, image upload dates, annotation history dates, annotations exported as CSV, blog post dates, newsfeed item dates, etc.

Unit tests are always good as well.