Kindergeld is paid backwards for six months and not a day more. Put in the month your entitlement began and this marks every month that is still within reach, every month that has already gone, and what each month of waiting costs you.
Your child's date of birth, or the day you started living or working in Germany, whichever came later. If you are not sure of the day, the first of the month is close enough.
Section 62 (2) of the Income Tax Act decides this by residence title, not by nationality alone.