Although this was clearly intended as the Christmas 1988 episode, it was not broadcast until August 1989, as NBC were using the series as a last minute replacement show to fill gaps in the schedule, much to Michael Landon's anger and which ended his thirty year relationship with NBC.
When Jonathan and Mark go thirty years into the future, Mark Gordon is surprised to still be alive in his 80's. Jonathan tells him that because he quit smoking, his health would get better and he would live into old age. In real life the opposite became true. Victor French, the actor who plays Mark Gordon, died six months later from lung cancer brought on by a lifetime of chain smoking, making his appearance in this episode his last performance. Also, because NBC decided to delay the broadcast of the show from the time originally intended by the producers, French would not live to see the broadcast of the last seven episodes. French did not know at the time of production he had lung cancer.
Chernobyl is Smith's shorthand reference to the Level 7 nuclear accident on April 26, 1986, 80 miles north of Kiev, Ukraine, in the former USSR. Over 30 people died then and the area is not safe to inhabit.
Three Mile Island is Smith's shorthand reference to the Level 5 nuclear accident on March 28, 1979, near Harrisburg, PA, where there was a partial meltdown and radiation leak.