String inputs should be validated and replaced by "today" values.

Summary: Ref T8060, String inputs should be validated and replaced by "today" values.

Test Plan: Open Calendar advanced search, enter "tea time" in date input control, open datepicker, "today" should be selected.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8060

Differential Revision: https://secure.phabricator.com/D13018