title: python datetime calculation not trimming to milliseconds
body: <p>Doing the following calculation to take 30 days of the current date using the date-time module. The calculation is correct but it's not trimming milliseconds to 3 digits. any idea how I can implement it?</p>
  <pre><code>from datetime import datetime,timedelta
  datetime_limit = datetime.today().strftime('%Y-%m-%d %H:%M:%S.%f')[:-3]
  datetime_limit = datetime.today() - timedelta(days=30)
  </code></pre>