how to - raddatepicker in column show month year only

[Geena]

Below shows month year as I want in the column but when you click the drop down date picker it shows days also. I need it show the month year picker control. How can I do this ? thanks
<telerik:GridViewDataColumn DataMemberBinding="{Binding CheckDate}"  Header="CheckDate" DataFormatString="{} {0:MMM-yyyy}" />