Barney thinks he can stay in the past as long as necessary, but first Ruf backs out and has to be returned, and then the time machine itself breaks down in the present day, taking hours to repair.
He can stay in the past as long as necessary. This is what Barney thinks. First Ruf backs out. First Ruf has to be returned. Then the time machine itself breaks down in the present day. Then the time machine itself is taking hours. This is to repair.