I feel like I'm going round the houses on this one.
In summary, I have a Location model which has opening hours, modelled as opens_at and closes_at in the DB.
All times are stored as UTC in the database.
If I create a location in New York with opens_at set at 08:30 in my form and it gets saved to the DB as 08:30 UTC, why would I need to worry about timezones? I don't want to display the time in local time because the user would then see opening hours of 03:30.
Am I overthinking this? Do I only need to worry about timezones if I have users in multiple countries attending the same event at the same time?
As this is about physical locations, it feels to me that it wouldn't be needed.
If I create a location in New York with
opens_atset at 08:30 in my form and it gets saved to the DB as 08:30 UTC, why would I need to worry about timezones?
Because 08:30 in New York is not 08:30 UTC. It's not even the same time for all dates.
So, if the location opens at the same time every day, then you must store it in local time. Otherwise you'll be off by an hour one way or the other following the next DST transition.
This can be an issue even for time zones that don't use DST, and even for non-recurring events, because the various governments of the world control the time zones within their borders. They can change whether to start observing DST or stop observing it, the dates and times that DST transitions occur, or their standard offset from UTC. Some give adequate notice when they make such changes, and others do not. There's always some non-zero chance that a given time zone might change its behavior between when you initially recorded the event and when the event takes place. If you recorded using local time, all you need to do is update the time zone data on your systems (which often happens automatically). However, if you recorded the event using UTC then you could be off when converting back after such a change.
In general, future events should always be stored in terms that the event is described - which is almost always in local time of some particular time zone. Store the IANA time zone ID (e.g., "America/New_York") with the location or event so that you can convert when you need to, but converting to UTC prematurely can lead to losing sight of the original information.
Save the "Always UTC" mindset for past or present events, such as timestamping a transaction.