Private Sub Filter()
        Cursor.Current = Cursors.WaitCursor
        Form_PlanningGuide.PlanningGuideDataDataGridView.Visible = False
 
        'Dim ci As New CultureInfo("en-US")
        Dim dt1 = DateTimePicker_StartDate.Value.ToString("yyyy-MM-dd")
        Dim dt2 = DateTimePicker_EndDate.Value.ToString("yyyy-MM-dd")
 
        'Form_PlanningGuide.PlanningGuideDataBindingSource.Filter = 
        ' String.Format(CultureInfo.InvariantCulture, "[Column_25] >= #{0}#" & _
        "And [Column_25] <= #{1}#", dt1, dt2)
        Form_PlanningGuide.PlanningGuideDataBindingSource.Filter = _
        String.Format("[Column_25] >= #" & dt1 & "# And [Column_25] <= #" & _
        dt2 & "#")
 
        Form_PlanningGuide.PlanningGuideDataDataGridView.Refresh()
        Form_PlanningGuide.PlanningGuideDataDataGridView.Visible = True
        Cursor.Current = Cursors.Default
    End Sub

Form_PlanningGuide.PlanningGuideDataBindingSource.Filter = String.Format( _
  "[Column_25] >= #" & dt1 & "# And [Column_25] <= #" & dt2 & "#")
 
=> Die Operation '>=' kann nicht an System.String und System.DateTime _
  durchgeführt werden.