Since #17553 DateTimeValidator has format option, which should cover all needs.

Other mentioned validators are very unflexible. For instance, we wanted to validate date is in d/m/Y format and found we could not use DateValidator for this, because there is no way to change the order of regex matches passed to checkdate().

Assuming upgrade path in user space is following

Date() -> DateTime(['format' => 'Y-m-d'])
Time() -> DateTime(['format' => 'H:i:s'])

This should also cover a need of failing validation when value passed in contains extra characters, same as current solution with regex:

>>> DateTime::createFromFormat('Y-m-d', '2018-08-08 20:00:00')
=> false
>>> DateTime::createFromFormat('Y-m-d H:i:s', '2018-08-08')
=> false