R1:02710ee779f4

Rename the `_phutil_date_format` function for consistency

Summary: Ref T5655. Depends on D11188.

Test Plan: `grep`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D11189