start is the local device date/time (first save) end is the last save time, normally also local timezone… But if a case was edited on server level, this is based on the server timezone.
Your situation can also happen, if date/time was changed on the local device (manually or automatically network-based).
Using the search function of this forum, please, will give you further hints, e.g. Multiple submissions randomly appeared all at once on server days to weeks after submitting - #4 by wroos.
