Managed to completely skip the pressure plate part (you can go around the container in 1983).
Having both time switching and a gravity gun creates a lot of potential for makng objects stuck inside each other. I made the giant board disappear this way. Not sure how you can go around this thing, perhaps it just needs a lot of careful level design.
The sounds and visuals are nice (and I like how different times are represented by different visual styles), but I think some ambient or background music would help.
Overall, nicely done!