I’ve been letting this roll around in the back of my mind all day and I think it probably makes most sense to use a global time value set to the equivalent of localtime() (or whatever the python version is). The timer never really needs to know it’s timezone since I very highly doubt that anyone is using one across timezones.
In the event someone is managing multiple timers that could be in different timezones I believe they probably are going to need to keep track of which timezone each timer is in anyway so as not to run into problems dealing with any local support staff that are likely in this scenario.
Since the underlying operating system is taking care of the system clock I doubt there is value in running on UTC time and adjusting for the timezone as there is on the OpenSprinkler that has to keep track of the RTC and NTP.