I'm just going to "fix" this by just preventing you from hiding/locking/defaulting these fields, since it doesn't seem useful to create a "New Event on January 3, 2013" sort of form anyway.

In the longer term there's probably some more-correct / more-general fix in this vein:

https://phabricator.wikimedia.org/rPHABdbc2ad3debdf1189cd93286ffad7861392c5c775

...but we can wait until there's a more direct issue with date controls before sorting that out. They may need to be adjusted to generate PhutilCalendarDateTime values anyway (e.g., for T11073 and internal transaction behaviors related to dates coming from ICS files and external APIs).