rval will be equal to the interval of the datetime variable. When
specified it can only be lower than the interval and step size of the input data.
See Details.</p></td>
    </tr>
    <tr>
      <th>start_val</th>
      <td><p>An object of class <code>Date</code>, <code>POSIXct</code> or
<code>POSIXlt</code> that specifies the start of the returned datetime variable.
If NULL it will use the lowest value of the input variable.</p></td>
    </tr>
    <tr>
      <th>end_val</th>
      <td><p>An object of class