You can also batch edit if you need to. Select the photos you want to change, click on "Photo" on the menu bar, then select "batch change" . You can change dates, titles, etc. from there for multiple photos.
The most recent iPhoto also has an option to "adjust date". This will leave most other info alone, just adjust the date (for instance if you just want to change the year, it will leave the month and time alond).