R1:fae0ec9220a7

Use more CalendarDateTime and fewer epoch timestamps in Calendar

Summary: Ref T10747. Moves away from getDateFrom() / getDateTo() and makes a few more date/time methods more consistent.

Test Plan: Created, edited, viewed events.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16663