How do I edit/add a birthday in the `people` app Cyanogenmod 11?

I have Cyanogenmod 11 M8. I keep my contacts on the phone, not sharing them in the cloud, I prefer to run only AOSP software and FOSS applications from F-Droid . Somehow I can't add/edit birthdays in the people app. Shouldn't this be a **basic** feature for a **people application** . I know they are somewhere because the birthday adapter shows the imported birthdays from my VCF file in the calendar. So how do I add a birthday?