TimeRangePicker times in nested HiddenSearch - how?

I had a simple XML Dashboard form with a company dropdown and a time range

<?xml version="1.0" encoding="UTF-8"?>
<form>
  <label>Excel Export dashboard Simple 2</label>
  <description/>
  <searchTemplate>sourcetype="completions_*" $company$</searchTemplate>
  <fieldset>
    <input type="dropdown" token="company" searchWhenChanged="true">
      <label>Select a Company:</label>
      <prefix>Company="</prefix>
      <suffix>"</suffix>
      <default>*</default>
      <choice value="*">All</choice>
      <populatingSearch fieldForValue="Company" fieldForLabel="Co_Name">
        <![CDATA[ | inputlookup rich-co ]]>
      </populatingSearch>
    </input>
    <input type="time" searchWhenChanged="true">
      <label>Select a time</label>
      <default>
        <earliestTime>-1mon@mon</earliestTime>
        <latestTime>@mon</latestTime>
      </default>
    </input>
  </fieldset>
  <row>
    <table>
      <title>Excel Completion Data</title>
      <searchString>sourcetype="completions_*" $company$</searchString>
      <earliestTime>$earliest$</earliestTime>
      <latestTime>$latest$</latestTime>
    </table>
  </row>
</form>

I have converted this to Advanced XML and it no longer works, it gives the red bar error Invalid earlier_time.

It seems it is caused by this

<module name="TimeRangePicker">
  <param name="label">Select a time</param>
  <param name="searchWhenChanged">True</param>
  <module name="SubmitButton" layoutPanel="viewHeader">
    <param name="label">Search</param>
    <param name="updatePermalink">True</param>
    <param name="allowSoftSubmit">True</param>
    <param name="visible">True</param>
    <module name="HiddenSearch" layoutPanel="panel_row1_col1" group="Excel Completion Data" autoRun="False">
      <param name="groupLabel">Excel Completion Data</param>
      <param name="latest">$latest$</param>
      <param name="search">sourcetype="completions_*" $company$</param>
      <param name="earliest">$earliest$</param>

In particular the latest and earliest param elements. If I change those to @mon and -mon@mon, it works OK but then it ignores the time range picker.

So, how do I get the time range picker values into the nested HiddenSearch?