 <code>Date</code>, <code>POSIXct</code> or
<code>POSIXlt</code> that specifies the end of returned datetime variable.
If NULL it will use the highest value of the input variable.</p></td>
    </tr>
    <tr>
      <th>by</th>
      <td><p>Only needs to be specified when <code>x</code> contains multiple
variables of class <code>Date</code>, <code>POSIXct</code> or <code>POSIXlt</code>.
Indicates which variable to use for padding.</p></td>
    </tr>
    <tr>
      <th>group</th>
      <td><p>Optional characte