title: Calculate months elapsed since start value in pandas dataframe
body: <p>I have a dataframe that looks as such</p>
  <pre><code>df = {'CAL_YEAR':[2021,2022,2022,2022,2022,2022,2022,2022,2022,2022,2022,2022,2022,2023,2023]
  'CAL_MONTH' :[12,1,2,3,4,5,6,7,8,9,10,11,12,1,2]}
  </code></pre>
  <p>I want to calculate a months elapsed columns which should look like this</p>
  <pre><code>df = {'CUM_MONTH':[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14]}
  </code></pre>
  <p>how can I do this?
  my starting month would be 12/2021 or 12/31/2021 (do not care about dates here I only care about the months elapsed). This is economic scenario data but the format of the source data is not in the way we need it.</p>