title: How to add last day of the month for each month in python
body: <p>I have my data in the following format:</p>
  
  <pre><code>final.head(5)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/CgD8P.png" alt="Head of the data, displaying sales for each month from May 2015"></p>
  
  <p>(Head of the data, displaying sales for each month from May 2015)</p>
  
  <p>I want to add the last day of the month for each record and want an output like this </p>
  
  <pre><code>transactionDate sale_price_after_promo
  05/30/2015  30393.8
  06/31/2015  24345.68
  07/30/2015  26688.91
  08/31/2015  46626.1
  09/30/2015  27933.84
  10/31/2015  76087.55
  </code></pre>
  
  <p>I tried this </p>
  
  <pre><code>pd.Series(pd.DatetimeIndex(start=final.start_time, end=final.end_time, freq='M')).to_frame('transactionDate')
  </code></pre>
  
  <p>But getting an error </p>
  
  <pre><code>'DataFrame' object has no attribute 'start_time'
  </code></pre>