Let me see if I can help... the dates of your photos are assigned to the picture by your camera (not by iPhoto), so make sure you're setting the internal date & time correctly when you change your batteries. iPhoto just reads this data when you import a roll (you can however edit it as you wish in iPhoto).
I'm not 100% sure of the problem you're having, but I think the solution might be in learning that iPhoto can keep separate dates fro your "roll" and for each actual photo.
So, your "rolls" might be dated correctly, but the photos within may be wrong.
Photos in "rolls" don't have to have the same date (or even close) to the date assigned to the "roll".
Check those.
You don't have to change the date one at a time. You can do it in a batch (as many as you want) by shift-selecting the pictures with the dates you wish to change, right-click (or cmd+click) and choose "Batch Change...". In the batch change window set the "Date & Time" to whatever you want, and let it go.