Make overnight windows belong to the right day
A checked weekday identifies the day an overnight shift starts.
Ordinary windows
A Monday 09:00–17:00 window applies to local Monday. If only Monday is checked, the same hours on Tuesday are outside the declared window. Equal start and end times are rejected because they are ambiguous; use 00:00–24:00 for a full local day.
Crossing midnight
A Monday 22:00–06:00 window begins on Monday night and continues through Tuesday before 06:00. The early Tuesday hours belong to Monday's enabled shift, not to Tuesday's checkbox. A meeting beginning Tuesday at 05:30 and lasting 30 minutes fits; one lasting 45 minutes does not.
Separate people, separate rules
Each person can have a different named time zone, window and set of weekdays. A person with no enabled weekdays has no availability in this model. Enter one continuous window per person; split shifts, dated exceptions, holidays and leave are not represented.
Use the all-candidates view when no full overlap exists. It ranks starts by the number of people whose whole duration fits, then by UTC start time. That is a transparent count, not a judgment about whose schedule should be moved. Ask participants before changing real plans.