How to access typeconverted elements declared in XAML?

I declared

<sys:DateTime> 2009/09/01 3:33:33 
</sys:DateTime>  in xaml via <ListBox xmlns:sys="clr-namespace=System;assembly:mscorlib" >

开发者_如何学CI'm doing this

DateTime newDate = ((DateTime)(MyListBox.Items.GetItemAt(1)));
newDate = DateTime.Now;

but sys:DateTime's value is not changing??

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DateTime is a value type. It will not exhibit reference semantics like you wish. You'll have to assign back to the original in order to update it.