You don't say which year, but 1969-12-31 would be the Unix date of zero (1970-01-01) adjusted for a western (negative) time zone.
The correction that happens after that would be the phone getting the correct time and date from the cellular system it is connected to.
If you are seeing this "zero" date in the first place, it would seem to suggest that your motherboard does not have a correctly functioning battery to maintain the internal clock, etc, while the main battery is temporarily removed.
02-18-2020 06:58 PM