What problem does this feature solve?
Currently when we use the datepicker e.g. in Nextcloud Calendar, it is often confusing to people that they first have to pick a date and then a time (even though they e.g. click on the part of the input which shows the time).
What does the proposed API look like?
Not sure about the API, but proposed solution for this would be to make it look like the datetime-local picker does on Chromium. That way people can pick whichever

That way people can set either date or time, or both at the same time, whichever people need, and it’s not a forced 2-step flow. :)