Yeah, I don't know the history here, though in thinking through this, it ocurred to me that of course computers might need to represent time before 1970 for some reason, hence the sign, but of course there is still the limitation of 32 bits. Time is one of those things that seems simple, but always seems to bite you somewhere - things like leap seconds or if Juneteenth counts as a business day always seem to throw a wrench in.