Diet Coke was not on the market until 1982; Rebecca was pregnant in 1980 (she should have been drinking a Tab).
During the flashback to 1980, while Jack is driving to his father's house, he passes a Friendly's restaurant. The chain was called Friendly and didn't add the apostrophe s until 1989.
When Jack is driving to his dad's house, it is supposed to be around 1981-82, however all the cars on the street are modern cars.