Fix displaying day numbers in calendar

Summary:
They were shifted by one because of calling $day->setTime(24, 0, 0).

I've tried to clone the date and get the epoch from the clone but it was crashing for some reason.

Test Plan: /calendar/

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2882