Interesting, though I am not really going far into time keeping, I just sync my computer clock with NTP servers, and check it by having a $10GPS unit ( could never get the BT working on it, so use a serial cable from an old Nokia phone to interface it) with the NMEA stream displayed on a terminal emulator, and manually parsing the time info out of the stream and using that to check the time on the PC. Interesting in how the altitude by me has errors from 30m to 100m with time, as the different satellites come into view and go. Generally it dithers around my true altitude of 71m AMSL and agrees reasonably well with the Google street view coordinates for my window. At work with the dongle in the window I am at 20m ASL. Can believe it as when I did some digging to find sewage piping there was salt water in the sand 4m down.