R1:5cf09f567a98

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…