This code works for me to increase or decrease one day at a time. Must be put in the KEY PRESS event.  Thought I'd share in case someone else wanted to try it.

Private Sub EventDate_KeyPress(KeyAscii As Integer)
    
    Select Case KeyAscii
    Case 43            ' Plus key
        KeyAscii = 0
        Screen.ActiveControl = Screen.ActiveControl + 1
    Case 45            ' Minus key
        KeyAscii = 0
        Screen.ActiveControl = Screen.ActiveControl - 1
    End Select
    
End Sub