title: Converting month number to datetime in pandas
body: <p>How do I transform a month number (float) to datetime in pandas with a format like 2010-01-01?</p>
  
  <pre><code>    date
  0   1.0       
  1   2.0       
  2   3.0       
  3   4.0       
  4   5.0 
  </code></pre>
  
  <p><strong>Expected output:</strong></p>
  
  <pre><code>    date
  0   2010-01-01       
  1   2010-02-01      
  2   2010-03-01      
  3   2010-04-01       
  4   2010-05-01
  </code></pre>