datepicker is not working after postback

$(document).ready( function () { $("#txtDate").datepicker({ dateFormat: 'dd MM yy' }).val(); });

<asp:TextBox ID="txtDate" TabIndex="8" CssClass="txtalign" BackColor="White" ClientIDMode="Static" ForeColor="Black" runat="server"></asp:TextBox>

for first time load its works fine but after a click on a another button (means after a post back) it stop working.
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