The gold standard is software designed to be run near the North and South Poles, like snowmobile and ICBM navigation systems, which need to be able to switch between all time zones quickly.

Why would an ICBM need to use local timezones at all?

I can't imagine any teams up their using more than one timezone anyways. Perhaps UTC + their home zone?

After a 15-year career programming, I'm pretty sure the only sane way to handle timestamps is everything UTC server side, and let the UI convert to localtime on display.

Yup. That’s also my solution. Sadly, we’ve had to deal with doing other things recently, too, as our software was run on servers with the timezone set to UTC, but the hardware clock being 3 hours ahead of UTC (this is common especially on old Windows installs).

The result was that the displayed time on the server was local time, but if we sent it to the client, we got 3h offset there as well.