What is the difference between "passed" and "past"?

Sorry if this is a “stupid” question, by the way. I’m getting a little insecure about posting questions now… :(

Okay, let me explain. This question isn’t as general as you may think it is. I know that “passed” is the past tense of “pass” and I understand the difference between that and “past”.

I’m talking about “passed” as an adjective. Both “passed” and “past” are adjectives and both are related to the word “pass”.

But in the sentence “we are past/passed the days when we…”, would it be “past” or “passed”? Or does it not matter?

I’m supposed to be a grammar/linguistics expert, but this one has me stumped. Maybe I’m just being a moron and I’m missing something crucial. But even the dictionary says “passed” is “often confused” with “past”.