Before non-removable battery became mainstream, most phones had a small internal battery to keep the internal clock ticking when the battery is being swapped. This is not unlike the CMOS battery found on PC motherboards.

iPhones does not need this backup power since the battery is always attached. Hence in case the battery becomes completely drained the internal clock will reset. I can't recall what the default time iOS resorts to after a total power loss but IIRC it was a date well into the 2000s.

It’s not that – but "waiting until the time isn’t negative anymore" fixes it.

I fix iPhones. The battery needs to be really, really dead before it'll fix the issue. When a battery is reconnected the internal date is reset back to 0 (Jan 1 1970) until the phone connects to a cellular network/NTP server.

Wouldn't that result in the same outcome though? I find it more likely that the clock is reset to a more "sane" time - like Laforet stated as being "well into the 2000s".

When I least had an iPod touch, pretty sure it reset to 1/1/07

When the iPod I am using as test device lost battery, it reseted to 1970, but some months later than january.