Season four establishes that the Upside Down seems to be frozen in time at the moment Will entered it in season one. When Nancy, Robin, Steve, and Eddie are looking for a stowed gun in Nancy's Upside Down bedroom, they instead find one of her diaries dated Nov. 6, 1983, at which point she has not hidden the weapon in her room (she doesn't hide a gun there until at least 1984 in two).
This plot line works to explain why the gang has to come up with another plan, but it brings into question how other characters interacted with objects that weren't introduced until after Will disappeared in season one. For example, when Joyce hangs Christmas lights and paints letters on the wall in hopes of communicating with Will in season one, it's unclear how Will knows where the letters are if it was past the date on which the Upside Down's timeline is frozen. In season four's Lite Brite scene with Dustin, Lucas, and Erica Sinclair (Priah Ferguson), the two gangs separated by the Upside Down yell to each other to locate the approximate location of the toy so Nancy can spell out her messages and communicate with them. The Upside Down's frozen timeline doesn't seem to mesh with these scenes.
