R1:559ba4e94435

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

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