This is not ideal, but based on the reports here it likely points to the Hero3+’s internal clock backup power no longer holding charge. These cameras use a small internal backup battery or similar retention method to keep the clock alive when the main battery is removed. If that backup has failed or is exhausted, losing the date/time after some time without the main battery is expected.

One reply notes this internal battery is not user-replaceable, so the practical fix is to contact GoPro service. Another user reports similar behavior on a Hero3 after a firmware update, but that’s anecdotal—firmware may coincide with the problem, but the more likely issue is failed clock backup retention.

As a workaround, keep some power connected whenever possible, such as swapping batteries quickly or using external USB power, to avoid clock resets.

So: no, it’s probably not normal for a healthy unit to reset that quickly, and yes, it suggests the internal backup for the clock is failing.