Google checkout API - trial periods

Question

I am using Google Checkout for recurring payments (beta feature i beleive) and I cannot find any information on what the XML I post to google should look like in order to implement a trial period.

At the moment I have something like:

<?xml version="1.0" encoding="utf-8"?>
<checkout-shopping-cart xmlns="http://checkout.google.com/schema/2"&gt;
  <shopping-cart>
    <items>
      <item>
        <item-name>My Item Name</item-name>
        <item-description>An Awesome Item You Should Buy</item-description>
        <unit-price currency="GBP">29.99</unit-price>
        <quantity>1</quantity>
        <subscription type="google" period="MONTHLY">
          <payments>
            <subscription-payment>
              <maximum-charge currency="GBP">29.99</maximum-charge>
            </subscription-payment>
          </payments>
          <recurrent-item>
            <item-name>My Item Name</item-name>
            <item-description>An Awesome Item You Should Buy</item-description>
            <unit-price currency="GBP">29.99</unit-price>
            <quantity>1</quantity>
            <digital-content>
              <description>Awesome Item</description>
              <url>http://www.mysite.com/yourawesomeitem&lt;/url&gt;
            </digital-content>
          </recurrent-item>
        </subscription>
        <merchant-private-item-data>MyPrivateItemDataHere</merchant-private-item-data>
        <digital-content>
          <description>Awesome Item</description>
          <url>http://www.mysite.com/yourawesomeitem&lt;/url&gt;
        </digital-content>
      </item>
    </items>
  </shopping-cart>
  <checkout-flow-support>
    <merchant-checkout-flow-support>
    </merchant-checkout-flow-support>
  </checkout-flow-support>
</checkout-shopping-cart>

This works fine - it bills each month, but I cant find any clues online as to what and where I should put something to include, for example, the first 20 days for free. Any Help?