Hi...
Here add reference ajaxcontroltoolkit.dll. And then write
<asp:textbox id="TextBox1" runat="server" xmlns:asp="#unknown"></asp:textbox>
<asp:calendarextender id="calext1" runat="server" targetcontrolid="TextBox1" format="yyyy-MM-dd" cssclass="calext1" xmlns:asp="#unknown"></asp:calendarextender>
It wil show year and monthly wise.
Thank u.