Format a Date Field of CalendarExtender

I have a TextBox control with CalendarExtender to pick a date; it gives me 11/6/2013 format when I select today's date.
How do I format this date to always show the 1st for each month? e.g. If I seleced 11/6/2013 then it should display 11/1/2013.
<asp:TextBox ID="txtStartDate" Width="150px" runat="server" ></asp:TextBox> <asp:CalendarExtender ID="CalendarExtender2" TargetControlID="txtStartDate" runat="server" />