We do all our internals in GMT/UTC so if the sum of time zone and wall-clock time is correct, it should keep working.

Here is a tester script to compare with our server:

So yes, if "UTC time" in the OS is wrong, we will transition at the wrong time.

We only use tz changes as a cue to adjust location (like for traveling between timezones), but cancelling this dialog is just fine too.