Remove user-independent date and time functions from Phabricator

Summary: These have been moved into libphutil.

Test Plan: Browsed Phabricator, didn't see a crash.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9907