title: Fill in Missing Dates with Merge in R
body: <p><strong>df1</strong>: has all dates from 2021-04-20 to 2022-05-15</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>Date</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>2021-04-20</td>
  </tr>
  <tr>
  <td>2021-04-21</td>
  </tr>
  <tr>
  <td>2021-04-22</td>
  </tr>
  <tr>
  <td>2021-04-23</td>
  </tr>
  <tr>
  <td>2021-04-24</td>
  </tr>
  <tr>
  <td>2021-04-25</td>
  </tr>
  <tr>
  <td>2021-04-26</td>
  </tr>
  <tr>
  <td>...</td>
  </tr>
  <tr>
  <td>2022-04-12</td>
  </tr>
  <tr>
  <td>2022-04-13</td>
  </tr>
  <tr>
  <td>2022-04-14</td>
  </tr>
  <tr>
  <td>2022-05-15</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p><strong>df2:</strong> Random and incomplete list of dates between 2021-04-20 to 2022-05-15</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>Date</th>
  <th>Group</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>2021-04-20</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-21</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-25</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-27</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-30</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-05-05</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-06-06</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-20</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-04-21</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-04-22</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-04-23</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-05-24</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-05-22</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-06-23</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-06-24</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-07-01</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-07-12</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-08-09</td>
  <td>1002</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p><strong>dfdesired</strong>: Want to fill in incomplete dates in df2 with dates in df1. I want to do this by group. This means 1001 should start at 2021-04-20 and end at 2022-05-15. 1002 should also start at 2021-04-20 and end at 2022-05-15.</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>Date</th>
  <th>Group</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>2021-04-20</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-21</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-22</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-23</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-24</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-25</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-26</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>...</td>
  <td>...</td>
  </tr>
  <tr>
  <td>2022-04-12</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2022-04-13</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2022-04-14</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2022-05-15</td>
  <td>1001</td>
  </tr>
  <tr>
  <td>2021-04-20</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-04-21</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-04-22</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-04-23</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-04-24</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-04-25</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2021-04-26</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>...</td>
  <td>...</td>
  </tr>
  <tr>
  <td>2022-04-12</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2022-04-13</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2022-04-14</td>
  <td>1002</td>
  </tr>
  <tr>
  <td>2022-05-15</td>
  <td>1002</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>Not sure how to do this. Any help would be appreciated.</p>