In my experience, people are uncomfortable with the term "sextuplet" because of its association with another English word which, as all right-thinking people know, must never be associated with babies.

By the way, according to Wikipedia, the only significant event on 1989 Jan 1 was the Montreal Protocol governing ozone-depleting chemicals going into effect. Though it does appear to be about 9 months after the debut of the "Fox Trot" comic strip, which is suspicious.