Only validate the date if it is required. Addresses the date part of #65.
Fixes https://gitlab.cern.ch/cara/cara/-/merge_requests/56#note_3935368. And implements back-button support for the time required flag.
Fixes https://gitlab.cern.ch/cara/cara/-/merge_requests/56#note_3935368. And implements back-button support for the time required flag.