title: Lubridate returns negative MM and SS
body: <p>Performing this basic operation in lubridate:</p>
  
  <pre><code>library("lubridate")    
  seconds_to_period(10 * 3600) + hours(24) - seconds_to_period(19.67 * 3600)
  </code></pre>
  
  <p>Results:</p>
  
  <pre><code>"15H -40M -12S"
  </code></pre>
  
  <p>I was expecting:</p>
  
  <pre><code>"14H 19M 48S"
  </code></pre>
  
  <p>Is it normal?</p>