It's very likely there are a bunch of photos in your iPhoto collections that are displaying the incorrect date and time when the photos were taken.
And this isn't just a problem when your photos won't sort chronologically. This will also be an issue for you every time you create a new Event or album inside of iPhoto and it constantly tries to identify them using the wrong date.
Maybe the date and time weren't set correctly in your digital camera before you took these photos. Or it's possible you scanned a bunch of prints or film negatives and they are still reflecting the dates and times when you actually scanned them.
Note: If you aren't seeing the vertical panel on the right side with the time and date information you see in the image above, you will need to first click on the Info button in the bottom right of the application to open this “Info” panel.
How to Change the Date and Time Metadata in iPhoto
Verifying the Changes Were Applied
You can see that your dates have been updated by selecting one of these photos or videos and reviewing the information near the top of the Info panel on the right of the application.
Additionally, you could also open one of these photos in a third party program that is capable of displaying this type of date and time information (technically called EXIF and IPTC metadata).
One way to do this is to select a photo in iPhoto, and then click File>Reveal in Finder to bring up either the “modified” (edited in iPhoto) or “original” (master) photo in a Finder window. Then right-click (control-click) on it and then select “Open With” and choose your application of choice.
Note to beginners: be careful with these images you are pulling up. These photos may fail to load correctly in iPhoto if you alter or — yikes — delete them. So be careful and just view them.
In this screenshot below, I've opened the photo I changed in this tutorial using a lightweight program I bought in the Apple App store called iExifer. I highlighted in blue all the places where the updated date and time are reflected in the metadata information.
The instructions and screenshots were created using iPhoto '11 v.9.5.