R1:0b233e461ec5

Consolidate some datetime code and add unit tests

Summary: See D6115.

Test Plan:
- Ran unit tests.
- Viewed reports.
- Created a countdown.
- Searched chatlog.
- Searched pastes by created date.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6116