How to restrict user for selecting end date less than start date in Ajax Calendar Extender ASP.Net

I am using AjaxCalendar Toolkit :

<asp:TextBox CssClass="form-control" ReadOnly="true" runat="server" id="StartTime"></asp:TextBox>                  <asp:CalendarExtender ID="Start_CalendarExtender" runat="server" Enabled="True" TargetControlID="StartTime">
 </asp:CalendarExtender>

 <asp:TextBox CssClass="form-control" ReadOnly="true" runat="server" id="EndTime"></asp:TextBox>                  <asp:CalendarExtender ID="Start_CalendarExtender" runat="server" Enabled="True" TargetControlID="EndTime">
 </asp:CalendarExtender>

And On Page Load i have done this to avoid selecting previoud dates in start date :

protected void Page_Load(object sender, EventArgs e)
    {
      Start_CalendarExtender.StartDate = DateTime.Now;
    }

Now i want to do that user cannot select end date less than start date , e.g if he choose 2-July-2014 ( as a start date) then he could not choose (1-July-2014) as End date , dates prior to Start Date must get disabled in End Date Calendar Extender