title: How to select data from a timedelta list
body: <p>I have a list like this. I'm wondering how I can pick out the entries that have less than 21 days of timedelta and I want to keep the index as machine_id. </p>
  
  <pre><code>machine_id
  13512886     0 days 00:00:00
  49645796     0 days 00:00:00
  58622574     0 days 00:00:00
  59850911   147 days 01:57:40
  61433282     0 days 00:00:00
  61538672   135 days 23:25:24
  61755561   259 days 05:49:49
  61876734   259 days 21:00:28
  61950006   180 days 04:42:39
  62508680    59 days 06:45:46
  62549967     0 days 00:00:00
  62643307     0 days 00:00:00
  62678486     0 days 00:00:00
  63761469     0 days 00:00:00
  64092088     0 days 00:00:00
  64194930     0 days 00:00:00
  64543161     0 days 00:00:00
  64720467    75 days 01:11:00
  65034386     4 days 02:16:30
  65118059     0 days 00:00:00
  65198554     0 days 00:00:00
  65347738     0 days 00:00:00
  65524977     0 days 00:00:00
  65556861     0 days 00:00:00
                    ...       
  
  95318844     0 days 00:00:00
  95318910     0 days 00:00:00
  95319088     0 days 00:00:00
  95319234   211 days 22:15:05
  95319894   163 days 20:18:56
  95320067     0 days 00:00:00
  Name: event_datetime, Length: 3984, dtype: timedelta64[ns]
  </code></pre>