title: Convert number of days since Jan 1 2000 into date format
body: <p>I have a column in my data as a number of days since Jan 1, 2000 (this is 0 day). How can I convert day number into date format (2000-01-01)?</p>
  
  <pre><code>Number_of_days    Date
  3536              2011-01-03
  3537              2011-01-04
  3538              2011-01-05
  3539              2011-01-06
  3540              2011-01-07
  3541              2011-01-08
  </code></pre>
  
  <p>I want to add a new column called <code>Date</code> similar to above one into my <em>df</em>. Number of days to Date above is just an example.</p>