Nobody would consider the present as negative. Maybe he meant that previous work had to be coded as negative, after the zero was set.

Or maybe it's a joke. We used to say that some guy has his ID number in roman numerals... or used spears for the matter.

I'm guessing that "Unix timestamps" were not used before Unix itself became widely used, which would be after the Apollo missions

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.