Embedded portable device with a teeny ARM processor. Sadly, no room for linux anything or even an RTC. Every time it connected to a phone, the phone would set its clock so the timestamps were somewhat close to being accurate.
However, if you swapped out the AAA battery and DIDN’T connect it to the phone at least once, all your subsequent readings would go back to zero epoch and would be forgotten 🤷🏻♂️