iPhoto changes dates of photos relatively. For example, I have two photos in an event. One has date 1-Mar and the other one 2-Mar. If I select them both and change the date to one day before, the new dates will be 28-Feb and 1-Mar. How do I change a bunch of photos to exactly the same date, regardless of their original date in iPhoto?
In the menu, select Photos → Batch Change (⬆+⌘+B). Choose "Date" from the select box, modify it, and all of the photos' dates will be adjusted to the date you set.