title: Average timedelta in list
body: <p>I want to calculate the avarage timedelta between dates in a list.
  Although the following works well, I'm wondering if there's a smarter way?</p>
  
  <pre><code>delta = lambda last, next: (next - last).seconds + (next - last).days * 86400   
  total = sum(delta(items[i-1], items[i]) for i in range(1, len(items)))
  average = total / (len(items) - 1)
  </code></pre>