This clarifies things:
* GcalScheduleValues contains the real data, based initially on the
GcalEvent.
* WidgetState's date_time_start / date_time_end may not be the same as
the real data, since the values get adjusted for display, as in the
case when handling all_day events.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/550>