You can only get birthday’s which have been shared with the user by his/her friends. All others are off limits. In some cases, this means that you may not be able to get the year depending on what each person has decided to share.
In relation to your previous question (which someone else linked to earlier), I would strongly recommend being very careful about what you are designing an app to do. And please make sure that it follows the Terms of Service. You can find them at https://developers.facebook.com/policy/.
solved Obtaining friends’ birthdays?