There is a bug in the Edge fw (probably in older Edge units too) since a year (when i noticed it first), if the unit put only in sleep (so not turned off completely) and power on , the timestamps are out of sync in the file (i used +Galileo setting here) with 6-8secs to the basis and not "calibrated" to the real GPS time:
Black line: Xiaomi MI8 track with GPSLogger (so should be accurate GPS time in the file) as basis
Red line: Fenix3 with +Glonass setting
Blue line: Edge830 with +Galileo setting
the BT comm. is disabled in both Garmin units
the previous day, i turned off and on the Edge830 completely, there the timestamps are identical with Fenix3: (same GPS setting in the units as above, the difference was only the Edge 830 complete turn off/on procedure. )
Can somebody examine it ? For me it looks like the realtime time display on Edge display (time of the day field) is accurate but into the FIT file it written with this delay somehow if the unit is put in sleep.