I have some photos I recently imported into iPhoto. They were originally scanned from colour prints, and for some reason iPhoto has the original creation date set to 16th July 3888. I can highlight the event, go to Photos -> Adjust date and time... and change the year to 1970, then click Adjust. When I go back to the event, the creation date is now set to 28th June 3820. If I repeat this process often enough, I can eventually set the date correctly to some time in 1970, but it takes a while and I have a few events with the same problem. If I try and change the creation dates of individual photos within an event, I get the same problem.