title: read in string as datetime object with 3 digits for millisecond
body: <p>I am reading in a string that is in a datetime format. However, instead of having 6 digits for the millisecond, it has only 3 with a letter Z at the end. How do I read in this string and make it a datetime object, and add 1 day, and write out this as a string in the above format i.e. 3 digits for the millisecond and a letter Z at the end. I tried the following code but not successful:</p>
  
  <pre><code>old_date= "2018-06-06T23:59:59.999Z"
  new_date = datetime.datetime.strptime(old_date, '%Y-%m-%d %H:%M:%S.%f%Z') + datetime.timedelta(days=1)
  print(new_date)
  </code></pre>