Add some DateTime handling related PHPDocs

Summary:
As I was fiddling with an exception handling invalid DateTime, add some PHPDoc
for the next time someone needs to understand this code.

Test Plan: Use get_class() to check parameters and return values.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25793