In what century did Olaf Geirstad-Alf  and Halfdan the Black live in?
Olaf Geirstad-Alf:  He was the son of Gudrød the Hunter and according to the late "Heimskringla", the half-brother of Halfdan the Black. Halfdan the Black: Halfdan the Black (Old Norse: "Halfdanr Svarti") (  810 –  860 ) was a ninth-century king of Vestfold.
ninth-century