In STM, Lex mentioned that the rocket left and took 3 years to get to earth (meaning that Kal_El was 3 years old when he landed) If you take that, It would place Kal landing on Earth about the same time that Chris Reeve was born. Chris was 24 or 25 at that time, so you add on 3 years and you get 27 years old in 1978. So STM firmly establishes itself in 1978. Also, Since a song is playing on the car radio from the 50's when Clark runs home to beat Lana and gang, that establishes 1978. However, Clark would have been 6 when that song was released and playing on the radio. Superman is supposed to be about 27 or 28 in the movie. Originally he was written as 30, but they settled on the date he landed on earth + 3 years. So technically it is dated as SII was to originally be released the year following STM because of Donner's vision. Actually, I think if he had been able to get both films in the can during the whole shoot, I believe S2 was supposed to come out Summer of 1979, or 6 mos after STM. Only due to the Salkind's B.S. was the movie not reshot and re-leased in the summer of 1981 (or December of 1980 for those living in Australia and the southern hemisphere market's, where the film was released 6 mos earlier) So STM and Donner's S2 are firmly locked into a year.
Another thing is a deleted scene in Air force 1 where the pilots reference the President and peanuts, so they are definately talking about Carter. Also, I believe it is during Carter's time that Air Force 1 switched from a 707 to a 747, so that dates it too. It could have been later, but I do believe as a kid I saw Carter's Air Force One being a 707, and Reagan's being the 747. It was definately before the Beaver attack incident as that would have been mentioned instead of the peanuts as the peanut's ribbing was pretty much used secondly after the supposed beaver attack that no one saw happen. So it takes place during Carter's term (1976-1980) However, there is E.G. Marshall playing president in the movie, but no name is given, just "Mr. President". Had it been taking place during the Reagan years, we would have heard references to his movie with the horse.
So in any case, the film happens in the 70's while Carter is in office, but before the beaver attack.