People do, this is true.
So far, Im finding it hard to pick out what it is that the OP’s app actually does.
Is it using the time in order to just ‘do stuff on a scheduled basis’, or does it report the local time as it does so?
And essentially Im saying use UTC as a guide to what the ‘time since…’ is
Then do something and tell people what the local time is
and there in lies the problem. Unless the rules in the current OS are correct you cannot do that.
and those rules won’t be correct for older versions of an OS, or until the vendors issue a new one…
but should California decide by November… I doubt Apple or Microsoft will update on that day … meaning the auto adjustment of all computers will be wrong for at least some period of “time” (pun intended)
I don’t know the answer but this problem must have already been solved. How do systems in Arizona handle it since Arizona does not recognize Daylight Saving Time (except for a few of the Indian Reservations) while the rest of the Mountain time zone does.
A little investigation on timezone names shows that most of the Pacific timezone is derived from “America/Los Angeles”. This means that if California does go all Pacific Daylight the rest of the timezone will have to pick a new baseline abbreviation.
because AZ is already built into the DST rules of all major operating systems… but California opting out is not
and Pacific Time zone is the entire West Coast of the United States and most of the western edge of Canada…
so California will need its own brand new Timezone 
There is discussion in Congress to eliminate day light savings time and President Trump has indicated he is also in favor of it. Might want to wait a while to see if it goes the way of the floppy disk.
Either way Congress still has to approve it… California voters just approved it a few months ago… Florida voters approved theirs well over a YEAR ago, and Congress has yet to act…
Sometimes it’s just great living in NZ where we have two timezones NZ & CHA and I can ignore the CHA timezone as less that 1000 people live there.
Core.Date abstracts us from these issues though and moves the problem down to the OS vendor level.
Taking this conversation into account I’ll probably add a clause to my EULA to cover the OS vendor not being agile enough to fix the problem in a timely fashion. OS’s not up to date are already covered.
As for the concept of daylight saving I love it! We should have it all year round which is the same as not at all I suppose. I don’t care about going to work in the dark in winter (but then it’s only a walk down the stairs :))