how to get timestamp from header

edit

Hi ROS fans..

I am new in ROS.. I try to make a simple node that subscribes timestamp from a messages with a header and a timestamp here..

http://www.ros.org/doc/api/sensor_msgs/html/msg/JointState.html

As Describe in this tutorial

http://ros.org/wiki/ROS/Tutorials/WritingPublisherSubscriber%28python%29,

I make a simple coding like this..

#!/usr/bin/env python 

import rospy 
from sensor_msgs.msg import JointState

def callback(data):
    rospy.loginfo(rospy.get_name() + ": I heard %s" % data.header)

def listener():
    rospy.init_node('listener', anonymous=True)
    rospy.Subscriber('get_timestamp', JointState, callback)
    rospy.spin()

if __name__ == '__main__':
    listener()

But I get this error..

 rospy.loginfo(rospy.get_name() + ": I heard %s" % data.header)
      TypeError: float argument required, not Header

why float argument required ? How to solve that problem..

Actually I am confuse with the Sensor_msgs/JointState, because the type of the header is std_msgs/header.

how to subscribe it from sensor_msgs/JointState to get timestamp ?

I hope someone can help me.. :) Thanks..