'>time_variable()</a></code> function to create the necessary variable if your data uses a <code>Date</code> variable for time.</p></td>
    </tr>
    <tr>
      <th>.d</th>
      <td><p>Number indicating the gap in <code>.t</code> between one period and the next. For example, if <code>.t</code> indicates a single day but data is collected once a week, you might set <code>.d=7</code>. To ignore gap length and assume that "one period ago" is always the most recent prior observation in the data, set <code>.d=