Since iDayZ reads all event data from your address book (Contacts) all you need to do is to update your contact information. You can do this either on the iPhone / iPod Touch or on your Mac/PC if you are syncing you data.

On the iPhone/iPod touch you need to go to a contact, click edit and from there to the Add Field selector. Towards the bottom of the screen you will see Birthday and Date to which you can add either/or. Start iDayZ back up and it should pick up the change.