One would assume about 4 years before the movie was released 😉
Seriously, though. The correct answer is the mid-1990’s. They had originally decided on the year 1996 but later deleted the references making it more vague.
“Flood” was released by Jars of Clay in late 1995 and “Twister” a popular movie from 1996 was supposed to be seen in the marquis of a movie theatre that Jamie and Landon walked by.
An early draft script had the story take place during the 1985-6 school year as indicated by the explosion of the Space Shuttle Challenger (Jan. 1986), the movie Ferris Buler’s Day Off, and numerous 80’s music references.