I wish that I could give you a definitive, specific date to answer this question; however, that is not possible. Some sources state that the events within the story are taking place in the early 1990's because that is when the book was published. Based on certain details within the text, placing the story that late in the 20th century does make some sense. The author doesn't provide readers with a specific date in which the story takes place, but readers can begin to narrow the date range down through Maniac's interactions with Earl Grayson. A key piece of information about the time setting of the story comes when Grayson tells Maniac that he pitched against the legendary Willie Mays just before Mays went on to play baseball in the major leagues.
The happiest story being the one about Willie Mays's very last at-bat in the Minor Leagues, before he went up to the New York Giants and immortality. Well, it was ok Grayson himself who had last crack at Mays, in the ninth inning of a game with Indianapolis -- and what did Grayson do! All he did was set the Say Hey Kid down swinging -- on three straight curve balls.
Willie Mays was born in 1931 and was pulled up to play for the New York Giants in 1951. That would put Willie Mays at age 20 when he faced Grayson. We are told that Grayson was 27 at the time he faced Mays at the plate. Then readers are told that Grayson survived in the minor leagues for another 13 years. That puts Grayson at 40 years of age when he retired from baseball. The year would have been 1964. Maniac doesn't know how old Grayson is, but Maniac and readers know that Grayson is quite old. If Grayson is 70 at the time the story takes place, then the story is taking place in 1994. That is just about the latest that I would place the story, and I might even consider suggesting that the story takes place a decade earlier. The reason that I think that is a strong possibility is because technology other than televisions is absent from the story. The kids in the story do not have or play with CD players or even portable cassette players like the Sony Walkman which debuted in 1979.