You're right, I used a slightly different approach, but yours would have worked as well. (yeah, I'm now the expert :) ).

Public Function Extract(fromDate As Date, toDate As Date, sp As String) As Recordset
    Dim sql As String
    sql = "select * from sp_aankopen('" & sqlformatDate(fromDate) & "','" & sqlformatDate(toDate) & "')"
    Debug.Print sql
    Set rs = New ADODB.Recordset
    rs.Open sql, conn, adOpenDynamic, adLockOptimistic, -1
    Set Extract = rs
    Set rs = Nothing
End Function

I see now what I did wrong. :)

Many thanks!