TL;DR: 09:00 is wall-clock time in a timezone, not a UTC instant. Resolve the offset in the business's zone and convert to UTC last, or you're an hour off across zones and grow a phantom slot on the DST change.
source → https://dashforge-ui.com/guides/availability-slots-timezones-dst
An availability engine looks trivial until it ships. It works on your machine, in your timezone, on an ordinary week. Then a customer in another timezone sees every slot an hour off, or the clocks change and a Sunday grows a slot that doesn't exist. Every one of these is the same mistake: treating a wall-clock time as if it were a fixed point in time.
"09:00" is not an instant
A working window — open 09:00 to 12:00 — is wall-clock time in the business's timezone. It is not a UTC offset and not a number of minutes from midnight UTC. 09:00 in a Paris salon is 07:00 UTC in summer and 08:00 UTC in winter, because Paris is UTC+2 under DST and UTC+1 otherwise. The same "09:00" maps to two different instants depending on the date.
