Actually, in the original Japanese dialogue, the narrator says "It's the close of the 20th Century," the Toho International dub specified "1999," which was carried over into the AIP version. Similarly, Dr. Yoshida's dialogue in the Japanese version isn't date-specific, either. He says "Around twenty years ago," so there's no concrete specifics, in which a viewer can accurately pinpoint dates for the creation of Monsterland. Nor do I think they should waste their time doing so, after all, it's only a movie.