href="namespaceboost_1_1locale_1_1period_1_1marks.html#a1536eaff4b2a880782128fc127f40d3a">period::marks::period_mark</a>&#160;</td>
          <td class="paramname"><em>p</em>&#160;</td>
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          <td>)</td>
          <td></td><td> const</td>
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  <td class="mlabels-right">
<span class="mlabels"><span class="mlabel">pure virtual</span></span>  </td>
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<p>Calculate the difference between 