CouchDb automatic timestamps

I'm implementing a message application using CouchDB. I want to apply timestamps to each message. I found some references indicating that I should use document update handlers for this. In place updates seem like the right thing. But where would I get a timestamp from? Is it in the req object somewhere?

{
  updates: {
    "in-place" : function(doc, req) {
      doc.timestamp = "???";
      var message = "set timestamp to "+doc.timestamp;
      return [doc, message];
    }
  }
}