title: DAX calculate index based on month
body: <p>in power query i have a table like this: </p>
  
  <pre><code>date       | month    |
  01/01/2017 | jan 2017 |
  02/01/2017 | jan 2017 |
  03/01/2017 | jan 2017 |
  ...        | ...      |
  01/02/2017 | feb 2017 |
  ...
  01/02/2017 | mar 2017 |
  ...
  01/01/2018 | jan 2018 |
  </code></pre>
  
  <p>now i want to add a calculated row that gives me an index based on month. 
  So for every new month the index needs to increase with 1. </p>
  
  <pre><code>index | date       | month    |
  01    | 01/01/2017 | jan 2017 |
  01    | 02/01/2017 | jan 2017 |
  01    | 03/01/2017 | jan 2017 |
  ..    | ...        | ...      |
  02    | 01/02/2017 | feb 2017 |
  ..    | ...        | ...
  03    | 01/03/2017 | mar 2017 |
  ..    | ...        | ... 
  13    | 01/01/2018 | jan 2018 |
  </code></pre>
  
  <p>Anybody knows how ?</p>