(1) past (plural pasts)
The period of time that has already happened, in contrast to the present and the future.
(2) past (adjective)
past (comparative more past, superlative most past)
Having already happened; in the past; finished.
(postmodifier) Following expressions of time to indicate how long ago something happened; ago.
Simple past tense and past participle of pass.
I think the best way to think about it, is that passed in the context of time is used wherever the word elapsed would be valid:
Time has passed / time has elapsed
Five minutes passed before either of us said anything.
past (except for The Past - meaning all time up until now) on the other hand is normally really a way of comparing two events, and generally means further than or later than.
It's time for bed. It's past eight o'clock!
The time now is half past three.