Ant: How can I subtract two properties (containing timestamps)?

I'm working on an ant script. In this particular part, I need to get the current month, as well as the prior month. I was thinking something similar to

<tstamp>
   <format property="thismonth" pattern="MMyy"/> <!-- 0210 by february 2010-->
</tstamp>

<!--I'd like to get 0110 (january 2010) here, but can't imagine how-->
<property name="priormonth" value="?">

I've been reading on property helpers, but I can't get what I need. What can I try next?

You can do it with a custom JavaScript scriptdef:

<project default="build">

    <target name="build">
        <echo message="Hello world"/>
        <setdates/>
        <echo message="thismonth ${thismonth}"/>
        <echo message="priormonth ${priormonth}"/>
    </target>

    <scriptdef name="setdates" language="javascript">
        <![CDATA[

            importClass(java.text.SimpleDateFormat);
            importClass(java.util.Calendar);

            today = new Date();

            cal = Calendar.getInstance();
            cal.setTime(today);
            cal.set(Calendar.MONTH, cal.get(Calendar.MONTH) - 1);

            priormonth = cal.getTime();

            fmt = new SimpleDateFormat("MMyy");

            self.getProject().setProperty('thismonth', fmt.format(today));
            self.getProject().setProperty('priormonth', fmt.format(priormonth));

        ]]>
    </scriptdef>

</project>