Private Function GetExpressionDate(ByVal dat As DateTime) As String
 
        'System.Globalization.DateTimeFormatInfo.InvariantInfo
 
        With dat
            Return "#" & .Month.ToString & _
                "/" & .Day.ToString & _
                "/" & .Year.ToString & "#"
        End With
End Function

Dim del_date As DateTime = Today.AddDays(-90) 'kritisches Datum
bs.Filter = "Bestelldatum < " & GetExpressionDate(del_date) 'Filter