title: How can I format timedelta for display
body: <p>My script calculate the difference in 2 time. Like this:</p>
  
  <pre><code>lasted = datetime.strptime(previous_time, FMT) - datetime.strptime(current_time, FMT)
  </code></pre>
  
  <p>It returns me a timedelta object. Currently, it gives me difference in seconds.</p>
  
  <p>How can I format it for display nicely? </p>
  
  <p>e.g. convert "121" to "00:02:01"?</p>
  
  <p>Thank you.</p>