I believe he started flying in his late 30s/early 40s so it would be around the early 1970s.

Edit: I just asked him (he’s at the dentist) and he said around 1980, guess I was wrong about when he started flying!

I would say no earlier than October 1977:

https://flightaware.com/resources/registration/N47943

That registration and the reference to cell phones not being available at that time puts the timeline somewhere between 1977 and 1987 or maybe 1990.

Then definitely before the DC ADIZ. It's kind of tough to make this mistake anymore.

It says that cell phones had not been invented yet:

> When we were ready to head back to Teterboro Airport, I got to a phone, [cell phones had not not yet been invented], and called Departure. [...]

That's not precise enough to really know. It could mean "before the 90s", "before 1983", or even "before 1973". The plane's registration is from 1977, so TFA can't mean literally "before cell phones were invented". Though I suspect it really means something like "before cell phones became widely available to private pilots", which presumably means "before 1990".