using AJAX
<ajaxToolkit:Calendar runat="server"
TargetControlID="Date1"
CssClass="ClassName"
Format="MMMM d, yyyy"
PopupButtonID="Image1" />
OR you can try
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</asp:ToolkitScriptManager>
<asp:TextBox ID="txtStartDate" runat="server"></asp:TextBox>
<asp:CalendarExtender
ID="CalendarExtender1"
TargetControlID="txtStartDate"
runat="server" />
Or try to google
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