ReadDate option outputs in the XML the most recent date-time the object has been marked as read by the current user (see http://forum.teradp.com/topic.asp?TOPIC_ID=641).

For example, if the 'img' object with id 6494 has been marked as read by the current user on May 19th at 11:57:52 PM (UTC), executing ../do.ashx?cmd=objsnav&ids=6494&options=ReadDate produces:

  <img nav:objectType="img" nav:readDate="2011-05-19T23:10:23.237Z" id="obj6494" . . .>

    . . .

  </img>

If the object has never be marked as read by the current user the attribute 'nav:readDate' won't be present.