Bibliography entries for online resources use a crdate attribute for the creation date of the pointer, on the <xptr> tag. I basically repurposed this as the last-accessed date for the resource (on the basis that I check all URLs when I create the pointer tag, so it's not really tag abuse). There's no such attribute for the P5 <ptr> tag, so we had to find another way to preserve that information in the P5 conversion code which produces the XML version accessible on the Website. This is what I've done (at the end of the template for handling <xptr> tags):

<xsl:if test="@crdate">
  <xsl:comment>Last accessed date: <xsl:value-of select="@crdate" /></xsl:comment>
  <xsl:element name="date">
    <xsl:attribute name="type">accessDate</xsl:attribute>
    <xsl:attribute name="notAfter"><xsl:value-of select="@crdate" /></xsl:attribute>
  </xsl:element>
 </xsl:if>

This is based on a suggestion from Lou Burnard on the TEI list, when I raised this issue in February.