I recently wrote the sentence, "I saw the deadline was a month passed," and have confused myself over whether it should be past or passed.
I believe it is passed—perhaps as an adverbial past participle?—because I am not referring to the past month, specifically, but that a month has passed. Also, if I take out "a month" I am left with, "I saw the deadline was passed" which I believe is more the meaning I seek (even if awkward) than "I saw the deadline was past". I am not confident, though.
Potentially relevant previous questions:
- "Past due" or "passed due"
- date has already passed OR date has already past? — This question, though, is simply a matter of the poster not knowing the meanings of past and passed, and was closed.