title: Reset the day of each month
body: <p>I have a column of a dataframe that looks like:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>Date</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>2017-05-13</td>
  </tr>
  <tr>
  <td>2017-05-13</td>
  </tr>
  <tr>
  <td>2017-05-13</td>
  </tr>
  <tr>
  <td>2017-05-14</td>
  </tr>
  <tr>
  <td>2017-05-14</td>
  </tr>
  <tr>
  <td>2017-05-15</td>
  </tr>
  <tr>
  <td>2017-07-02</td>
  </tr>
  <tr>
  <td>2017-07-03</td>
  </tr>
  <tr>
  <td>2017-07-03</td>
  </tr>
  <tr>
  <td>2017-07-04</td>
  </tr>
  <tr>
  <td>2018-01-14</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>I want to reset the day of each month:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>Date</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>2017-05-01</td>
  </tr>
  <tr>
  <td>2017-05-01</td>
  </tr>
  <tr>
  <td>2017-05-01</td>
  </tr>
  <tr>
  <td>2017-05-02</td>
  </tr>
  <tr>
  <td>2017-05-02</td>
  </tr>
  <tr>
  <td>2017-05-03</td>
  </tr>
  <tr>
  <td>2017-07-01</td>
  </tr>
  <tr>
  <td>2017-07-02</td>
  </tr>
  <tr>
  <td>2017-07-02</td>
  </tr>
  <tr>
  <td>2017-07-03</td>
  </tr>
  <tr>
  <td>2018-01-01</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>So each month should start by the first day of the day of the month. Days always follow each others.</p>
  <p>Thank you.</p>