The hour that doesn't exist: time zones, DST and the appointment that moved itself
A technician stood outside a locked building at seven in the morning because a recurring visit had been expanded into absolute instants six weeks before the clocks went back. The three kinds of "when" that don't convert into each other, why a recurrence rule is not a list of timestamps, what Odoo hands you over the API, and how to test a bug that only reproduces twice a year.