I always thought they sync with GPS time since most are already connected with the GPS navigator.
I will check that on my EDM930, the manual doesn’t say anything about this.
My EDM700 time is anything up to an hour off, after a few years. It means that if you are looking for a particular event in the data, it is hard to find it.
Crystal controlled clocks will drift, especially cheap ones.
It would be easy to bring in the GPS data stream, extract the exact UTC time from that, and adjust the clock as necessary. Does any engine monitor do this?