Rename the _phabricator_time_format function for consistency

Summary: Ref T5655.

Test Plan: grep

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D11187