The computer was locked with a code set, but the computer itself now shows "this Mac is locked. Try again in xxxxxxxx minutes" and we are unable to use pin to unlock computer. If a lock code is set on a Mac and the battery dies, the try again timer resets epoch time back to January 1, 1970.

In order to get the Mac back to the correct time, it needs to be reconnected to internet. In this situation the only possible way is with a hard wire connection (note: USB-C to ethernet adapter can be used).