<?xml version="1.0"?>
<page>
  <!-- invalid types -->
  <date dur1='2001'      dur2='-P1Y'/>
  <date dur1='P1M'       dur2='2000-01-01'/>
  <!-- cannot express result -->
  <date dur1='P11M30D'   dur2='-P12M'/>
  <date dur1='P11M'      dur2='-P12D'/>
  <date dur1='P1000Y11M' dur2='-PT9S'/>
  <date dur1='-P1000Y'   dur2='PT9S'/>
</page>

