title: How can I do a sequential count based on column value and timestamp in pandas?
body: <p>I would like to be able to add a column which counts rows in order based on a value. For example, below are three different people with records that have a timestamp. I want to count the order of records based on the PersonID. This should restart for every PersonID. (I am able to do this in Tableau with Index() but I want it part of the raw file too)</p>
  
  <pre><code>&gt; PersonID,             DateTime,             Order,     Total
      a226           2015-04-16 11:57:36          1          1
      a226           2015-04-17 15:32:14          2          1
      a226           2015-04-17 19:13:43          3          1
      z342           2015-04-15 07:02:20          1          1
      x391           2015-04-17 13:43:31          1          1
      x391           2015-04-17 05:12:16          2          1
  </code></pre>
  
  <p>If there is a way to subtract the DateTime as well? My way would be to only select Order 1 as a dataframe, then only select Order 2, then merge, then subtract. Is there a way to do it automatically?</p>