title: Count number of months since start in a data frame
body: <p>I have a data frame <code>df</code> which contains data from a car selling company. The data frame contains the dates and the number of sells for specific dates. Each salesperson has a <code>staff_id</code>. A dummy <code>inital_sell</code> indicates which day is the first working day of the person. </p>
  
  <p>Now I want to add a column <code>months_since_start</code> which adds for each day the month since the person started to work. Then I can use the <code>sells</code> and the <code>months_since_start</code> columns to plot the mean sells for each month since the salespersons started to work (sells for the 1st month for each salesperson, in the 2nd month...). As some days and months are missing (e.g., during holidays as shown in the bottom part of the example), I can't simply add a sequence to get <code>months_since_start</code>.</p>
  
  <pre><code>date        year    month   staff_id   sells  initial_sell   months_since_start
  2014-11-11  2014    11      1          3      1              1
  2014-11-12  2014    11      1          1      0              1
  2014-11-14  2014    11      1          1      0              1
  2014-11-15  2014    11      1          2      0              1
  ...                     
  2014-12-10  2014    12      1          2      0              1
  2014-12-11  2014    12      1          1      0              2
  ...                     
  2014-12-23  2014    12      2          1      1              1
  2015-02-02  2015    2       2          4      0              2
  2015-02-03  2015    2       2          1      0              2
  ...                     
  2015-03-23  2015    3       2          3      0              4
  ...
  </code></pre>
  
  <p>Can someone help me how to get the <code>month_since_start</code> column?</p>