code: function(callback) {
          if (parseInt(config.clockSkew, 10) > 0) {
            var now = new Date().toSerialNumber(),
                sent = res.parsedHeader.content.date;
            if ((now - sent) > config.clockSkew * 1000) {
              callback('date too old');
            } else {
              callback(null);
            }
          } else {
            callback(null);
          }
        }
docs: verify the date