Change latest and earliest saved timezones on all day events to php official timezones instead of guessing GMT offsets

Summary: Ref T8021, Change latest and earliest saved timezones on all day events to php official timezones instead of guessing GMT offsets

Test Plan: On different versions of php, create and save all day event in various timezones without errors.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8021

Differential Revision: https://secure.phabricator.com/D12772