Countdown - use better date control

Summary: Fixes T3576

Test Plan: made a countdown and it looked right on view. edited it and it had the right values pre and post edit.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T3576

Differential Revision: https://secure.phabricator.com/D8754