On Thursday, March 4, 2021 at 4:56:28 AM UTC-5, Tim Newport-Peace wrote:
IF the GPS transmissions that we use included data on which the current
epoch is, there would not be a problem, but they don't.

Save a couple of bits in the data packet, lose old equipment... But, is it possible to tell the GPS module what the current date and (approximate) time is? I mean, as a configuration message from the enclosing device. That depends on the GPS module of course. But if they don't build that into the module, that's planned obsolescence. If it is available, but the surrounding device (logger, vario...) doesn't use that feature, ditto. If you think that such a feature would be a burden on the module development, ahem, take a look at the uBlox documentation PDFs, they are hundreds of pages long.

This is a lot like the story with digital thermostats I use in my house. I used to have the typical thermostats that need batteries to maintain the real-time-clock, and the program memory, during a power outage. After some searching I did find a few models that do not use batteries at all, store the program in nonvolatile memory, and after a power outage require setting the clock manually. Why are such models so rare?