Have you tried to restore to original (if you've edited them)? Also, try to open them in Graphic Converter. I did with some photos that I edited, and I can see the original date and time.
I saw the posts in Apple's forums, and looked into it a bit.
I'm getting quite annoyed at iPhoto for these bugs. It's not rocket science that we're talking about...it would be a fundamental issue, IMO.
GL Linamen.
All I ever really needed to know, I learned from watching Star Trek.