problem implementing calender without using jquery

I have a calender with dropdowndown list to select month and year. i didnt use jquery. I dont have any error but the page gets reloaded whenevr i click month, year from dropdown and date from calender.  though values are corrected added reloading is disturbing. what should be done to to stop this?  pls help in  code.
Aspx code:
<tr> <td> <asp:Label ID="lblExpiry" Font-Bold="true" Font-Size="Larger" runat="server">Expiry</asp:Label> </td> <td> <asp:TextBox runat="server" ID="txtExpiry" /> </td> <td> <asp:ImageButton ID="ImageButton2" runat="server" ...