ASP.NET Calendar - Date picker and postback

Click on the calendar image near the textbox.
After you select a date press the "Postback" button.
Click on the calendar image again, the previous selected date is still selected in the calendar.

    Select date
 
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ASP.NET

<ASP:TextBox runat="server" ID="txtDate"></ASP:TextBox>

<obout:Calendar runat="server"
                         DatePickerMode="true"
                         TextBoxId="txtDate"
                         ShowTimeSelector="true"
                         DateFormat="MM/dd/yyyy hh:mm:ss">
</obout:Calendar>