title: Excel add serial number column based on another column grouped
body: <p>We have thousands of rows of data in the following format</p>
  
  <pre>
  <code>| Name       | Born On     |
  |:-----------|------------:|
  | Tom        |      Jul 15 |
  | Harry      |      Jul 15 |
  | Katy       |       Oct 2 |
  | Britney    |       Oct 2 |
  | Miranda    |       Oct 2 |
  | Victoria   |       Oct 2 |
  | John       |      Sep 30 |
  | Warren     |      Sep 30 |
  | George     |      Sep 30 |
  </code>
  </pre>
  
  <p>I would like to add a serial number based on the "Born On" column as follows</p>
  
  <pre>
  <code>| Name       | Born On     | Serial Number
  |:-----------|------------:|--------------:|
  | Tom        |      Jul 15 |  1
  | Harry      |      Jul 15 |  2
  | Katy       |       Oct 2 |  1
  | Britney    |       Oct 2 |  2
  | Miranda    |       Oct 2 |  3
  | Victoria   |       Oct 2 |  4
  | John       |      Sep 30 |  1
  | Warren     |      Sep 30 |  2
  | George     |      Sep 30 |  3
  </code>
  </pre>
  
  <p>The "serial number" needs to be grouped by "born on" date.
  Please help. Thanks.</p>