how to use property in echo

I want to dynamic generate a nant file for nant task use. I use echo file to 
generate. But property change to value that I don't want.for example:
<echo file="test.xml">&lt;property name=&quot;generateTime&quot; 
value=&quot;${datetime::now()}&quot;</echo>

test.xml would become like 
<property name="generateTime" value="2010 12 3 18:23">
How can I let it become below
<property name="generateTime" value="${datetime::now()}">
Thanks.


      

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