In the 1966 sequence, George refers to voting for Barry Goldwater, because his son has just died in Vietnam and says that they had received the news at the most recent 4th of July sometime in the last few months. Goldwater was a Presidential candidate in 1964 - two years before. He would have voted for Goldwater two years before his son died.
The morning of the first encounter, Doris goes into the bathroom to take a bath. The sheet she has wrapped around herself keeps changing positions. It covers both shoulders when she is outside and only one when she is in the bathroom.
Throughout the movie, there are several times where the couple places logs in the fireplace and starts a fire. During the last meeting, George starts the fire and places the screen back in front of the fireplace. Within seconds, the fire is blazing strong.
The intervals are mostly 5 years, except after 1966 it skips to 1972, as this marks their 21st anniversary. However in the 1977 meeting, George brags that he saved the marriage between Doris and Harry "6 years ago", that is,1971, not 1972 which was the actual meeting in which George pretended to be a counseling priest.