Force date/time preferences to valid values

Summary:
Fixes T8601. To reproduce the problem:

• Set your time preference to "" (the empty string). This isn't possible from the modern UI, but can be done with "Right Click > Inspect Element", or users may have carried it forward from an older setting (this is the case with me and @hach-que on this install).
• Load Calendar with some events.
• This parses an epoch, which sets valueTime to "" (since there are no format characters in the preference) and then getEpoch() fails because strlen($time) is 0.
• Since getEpoch() failed, getDateTime() also fails.

To fix this:

• Only permit the date and time preferences to have valid values.

Test Plan:

• Loaded page before patch, saw fatal.
• Applied patch.
• No more fatal.
• Viewed tooltips, dates/times, dates/times in other apps.
• Changed my preferences, saw them respected.

Reviewers: lpriestley

Reviewed By: lpriestley

Subscribers: epriestley, hach-que

Maniphest Tasks: T8601

Differential Revision: https://secure.phabricator.com/D13346