Booking systems look simple until two people click the same slot at the same second. The interesting engineering is all in the race conditions and the refund edge cases, not the calendar UI.
This build handles reservations with database-level locking on the slot, integrates a local payment gateway, and emails invoices automatically. The admin side shows a calendar view where overlapping bookings are impossible to create by accident.
Staff went from spending an hour a day on booking admin to almost none.