Passed and past are usually easy to use. Passed is the past tense and past participle of the verb pass. Past is (1) a noun meaning the time before the present, and (2) an adjective meaning completed, finished, no longer in existence, or in the past. But the words are occasionally confused, especially where past is an adjective. For example, this writer uses passed where past might work better:
One of those proposed fees, though, is a little different than in years passed. [Fox 12 Idaho (article now offline)]
This passed sort of makes sense because the years have indeed passed. But past, where it means completed, finished, or in the past, is the more conventional word in constructions like these, and while passed might make logical sense, some readers will see it as a misspelling. These are more conventional:
The bounce on Centre Court is slower and higher than it was in years past. [Telegraph]
As in past years, the Moo Brew stout has had a lengthy maturation in wood. [Sydney Morning Herald]
The ghosts of golden generations past hover disapprovingly, grumping from the broadcast booths. [New York Daily News]
Even in its reincarnation, Tahrir Square has kept the ebullience of months past. [New York Times]