I had to change it to
"=DateAdd('d',-1,DateAdd('m',9," & Me.datEffectieveStartdatumIn & "))"
but the result is not what was expected: (01/01/1900)
So I changed it to
"=DateAdd('d',-1,DateAdd('m',9,#" & Me.datEffectieveStartdatumIn & "#))"
And now it works.
Thank you for your help.