The date of the homo reptilian awakening depends upon the Doctor and Eldane speaking precisely. The year 3020 is never mentioned in the narration in Cold Blood. Rather, the Doctor and Eldane toss around the figure of "a thousand years", in the dialogue and Eldane's narration. Since the date of the story is explicitly 2020, then 3020 would, of course, be a thousand years from that. It seems reasonable to assume that the return of the homo reptilia would be exactly one thousand years later, as the revivification process was automated by computer.