Conversation so far:
Where is the team Ajax located?
CANNOTANSWER.
So he was a manager for Ajax?
On 11 May 2016, De Boer announced his resignation as manager of Ajax following a disappointing season when Ajax again lost out on the Eredivisie title to PSV.
What else did he do with Ajax?
De Boer has now won a total of nine Eredivisie championships with Ajax as a player and manager.

Question: When was he hired as a manager?
Self-contained rewrite: When was Frank de Boer hired as a manager?