Having two UNIX_EPOCH constants is a potential source of confusion

[ssokolow]

While trying to get comfortable with SystemTime, I found myself extremely confused because it appeared that UNIX_EPOCH was usable, despite being marked unstable.

It wasn't until I checked bug #49502 that I finally realized what I'd repeatedly overlooked: What was being successfully used was std::time::UNIX_EPOCH rather than std::time::SystemTime::UNIX_EPOCH.

The simplest way to head off this kind of confusion would be to have the documentation for the two UNIX_EPOCH definitions mention each other's existence.

[Dylan-DPC]

the constant has stabilised now, and they have the same documentation so this is less of a problem now.