And I'm wondering why would you ever want that?
The OP doesn't, as that is the whole point of this thread.
Surely it makes more sense just to leave the system running with its current clock time, which may be slowly drifting away from true time, but will be much closer than setting it to 00:00 01 Jan 1970 UTC.
There is the issue that if you have no internet connexion and no working transponders then the time set on the box has little significance, given that it can't do anything useful anyway, but I see no point in just discarding an approximated (and close) time just for the sake of it.