To be clear, you enter 1987 in night 7 by setting the difficulty of each character accordingly. (Freddy at difficulty 1, Bonny at 9, Chica at 8 and Foxy at 7). You unlock night 7 by beating night 6.
The only reason I tried 1987 is because it was an easter egg in the original game.
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In night 3 or 4, they tell you a bunch of parody dates. The sounding of 1983 (?) is a third code that gives you plushies. I don't remember the year of the event, but it's the sounding one.