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