<?xml version="1.0"?>
<page>
  <!-- valid date/times -->
  <sum>
    <date dur='PT1M'/>
    <date dur='PT2M'/>
  </sum>

  <sum>
    <date dur='P1DT2H'/>
    <date dur='PT3M'/>
    <date dur='PT4S'/>
  </sum>

  <sum>
    <date dur='PT20M'/>
    <date dur='PT20M'/>
    <date dur='PT20M'/>
  </sum>

  <sum>
    <date dur='PT01S'/>
    <date dur='PT02S'/>
    <date dur='PT04S'/>
    <date dur='PT8S'/>
    <date dur='PT16S'/>
    <date dur='PT32S'/>
  </sum>

  <!-- Base case: sum of 1 node -->
  <sum>
    <date dur='P1M'/>
  </sum>

  <!-- Non-normalized input -->
  <sum>
    <date dur='PT70M'/>
  </sum>

  <sum>
    <date dur='PT70M'/>
    <date dur='P13M'/>
  </sum>

  <!-- Negative durations -->
  <sum>
    <date dur='PT70M'/>
    <date dur='-PT30M'/>
  </sum>

  <sum>
    <date dur='PT1H10M'/>
    <date dur='-PT30M'/>
    <date dur='-PT30S'/>
  </sum>

  <!-- Zero sum -->
  <sum>
    <date dur='PT1M'/>
    <date dur='-PT1M'/>
  </sum>

</page>
