It sounds like maybe the dates in the EXIF data are off? (the date that your digital camera tagged the image with when the picture was taken).
I think I am on an older version of iPhoto (still running Panther), but I doubt Apple has done away with the Batch Change command.
If you select the photo where you want to change the date, then select Photo>Batch Change (or just hit Command-Shift-B), iPhoto will let you wholly change the date, or roll the date of the image forward or back.
There's also a freeware program called PhotoInfo (
http://www.macupdate.com/info.php/id/12156) ...that will do the same thing.
I played a similar game when, after I bought my new camera, I mistakenly set the date ahead by one month (so I was able to roll all of the pictures back 1 month).
These batch change methods seem "made" for correcting large groups of data, but if you pick your pictures by "groups" of when they were taken (however small that group might be) and set your dates accordingly, they might give you a good start.