Since some time the dates in my iPhoto library have all changed to the day my library was created. The pictures still appear chronologically in the events screen but they all show the creation date (of the library not the picture) under the event. If I try to change the date and time I can only change the time of the picture the date is fixed on January 5th, 2013.
Repair Disk Permissions for the hard drive where your iPhoto library is. You can do this from the
Disk Utility app.
This might (or might not) solve your problem about not being able to change the date of your current pictures. It will probably won't restore your original dates.