Passed or Past?

"Not a day has past, but what I think of him."  That's the sentence that prompted this blog post.

Passed is a verb meaning to move on or move by.
Past is an adjective, adverb, or preposition meaning earlier.

Example:  "That car has passed me in the past."

Now, which of these is correct?

"Not a day has past, but what I think of him."
"Not a day has passed that I don't think of him."

Yeah, I know I changed the last part of the original sentence.  I don't have time to get into why I'd never use "but what" in that sentence.