Conpherence - fix calendar bugs part deux

Summary: I needed to port my extremely clever "last sunday as of tomorrow" trick to the display layer. Also found a fun bug in testing where +N days was changing it to 1:00 AM from 00:00 AM with my timezone configuration. Presumably all sorts of whacky hyjinx ensue when you modify DateTime and you need to re-specify the timezone after to get it to work

Test Plan: verified that Today, SUNDAY, we see TODAY -> Saturday and it all looks good. Verified midnite -> just before midnight status events span but a single day.

Reviewers: epriestley, chad

Reviewed By: chad

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6114