Fix an issue with date parsing when viewer timezone differs from server timezone

Summary:
The way DateTime works with epochs is weird, I goofed this by having my server/viewer timezone the same and not noticing.

Also fix an issue where you do ?epoch=... and then manually fiddle with the control: the control should win.

Test Plan:

• Set viewer and server timezone to different vlaues.
• Created a countdown using ?epoch=....
• Created a countdown using ?epoch=... and fiddling with date controls.
• Created and edited a countdown using date/time control.
• Poked around Calendar to make sure I didn't ruin anything this time (browsed, created event, edited event).

Reviewers: lpriestley, chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15680