hi! sorry for the late response, it’s been like 4 years and i needed to play thru the game myself.
you need to get bad ending B (force through that big door) and get the year 1978 from it. use that as the code for the little box on the hallway, then make sure to see the events in the infirmary and church.
make sure to interact with the big door after and do NOT break in. the lil box gives you worn note and last note I, then you get last note II. rest of the game should be linear and you’re locked into the good timeline.