The Birthday Calendar is managed by the Operating System and automatically filled with the dates-of-births of your
Contacts from the Contacts app. To add a new birthday to this calendar, please set the date-of-birth for the appropriate contact.
- Go to the Contacts App and select the contact to add a birthday to. Now tap "Edit".
- Scroll down and tap on "Add field"
- Scroll down and select the "Birthday" field
- Set the Birthday and tap on "Done"
Birthdays without a year
Apple allows you to enter birthdays without specifying a year.
This results in all kind of errors varying from duplications or showing birthdays on the wrong date.
So, please always enter a year (even if you don't know the exact one).
You can ask Apple to make the year an obliged field by giving them feedback here: