When converting an absolute datetime to an absolute datetime, just clone it

Summary: Ref T11801. Previously, we would go through the generic implementation which incorrectly tagged all-day datetimes as UTC.

Test Plan: Imported a French holiday. See next changes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11801

Differential Revision: https://secure.phabricator.com/D16813