title: how to subtract two Solar Hijri column date
body: <p>I have a data frame like this:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>index</th>
  <th>ft</th>
  <th>rt</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>0</td>
  <td>1401/04/21 13:36:00</td>
  <td>1401/04/21 13:09:16</td>
  </tr>
  <tr>
  <td>1</td>
  <td>1401/04/21 14:00:00</td>
  <td>1401/04/21 13:00:00</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>these column dates are based on &quot;the Solar Hijri calendar&quot;.
  and when I use</p>
  <pre><code>type()
  </code></pre>
  <p>for values in these columns, it shows me that they are as strings.
  how can I subtract these columns ?! (ft - rt)
  Any help would be greatly appreciated!</p>