I'm trying to use EXIF data from my photos, and I discovered the wonders of the tag "EXIF GPS GPSTimeStamp" (a timezone-independent timestamp).
However, I notice that it's often different from "EXIF DateTimeOriginal". Usually by a second or two, but I've seen up to 211 seconds difference. The GPS time is usually, but not always, behind (earlier) than DateTimeOriginal. The DateTimeOriginal seems to be the accurate one in the case of a discrepancy. I've checked this for a few hundred photos from 3 different phones.
Does anyone know when, exactly, "GPSTimeStamp" refers to on most phones?
Since GPS signals encode timestamps from literal atomic clocks in space, I assume if the phone receives any signal at all, it will be accurate to less than a second. So that can't be the source of the error.
My current theory is that the "GPSTimeStamp" tells you the time of the GPS fix described in the other GPS tags, not the time the photo was taken. That would explain why it's usually earlier than the DateTimeOriginal. Is that correct?