Move date and time utility functions to libphutil

Summary: This moves the date and time utility functions that are not user-dependent into libphutil.

Test Plan: Generated library maps and viewed Phabricator instance.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9906