title: Multiply some amount with the total elapsed months since a specific month in google spreadsheet
body: <p>I have something like this</p>
  
  <pre><code> A             B               C
  1 Cost    Start Date      Total to date
  2 33      Aug 1st 2015        x
  3 12      Aug 1st 2015        y
  .  ..        ....                 ..
  
  n           MAIN TOTAL       x+y+...
  </code></pre>
  
  <p>Here are the formulas I am interested in:</p>
  
  <pre><code>month = number of months between aug 1st 2015 and jan 1st 2016, witch is 5 - counting only the beginning of the month
  
  x = A * month
  </code></pre>
  
  <p>What I'm not sure about is how to get the difference between today's month and the one specified in column <code>B</code></p>
  
  <p>Add on:</p>
  
  <p>The date format is not set in stone, can be anything that is easy to calculate, i can store the readable format in a different column if needed</p>