title: subtract two string objects pandas
body: <p>I am trying to subtract two columns in a pandas dataframe they are in the format dd/mm/yyyy I tried doing </p>
  
  <pre><code>data['end_date']-data['start_date] 
  </code></pre>
  
  <p>but the error returned was: unsupported operand type(s) for -: 'str' and 'str' </p>