Instead of changing the bag file time stamps, you should use simulated time.
See http://www.ros.org/wiki/Clock.

Short version:
 - Before launching any nodes, set parameter /use_sim_time to True.
 - Use the --clock option of rosbag play.

Then rosbag will publish the simulated time on the /clock topic. All nodes
using ROS time will use the simulated time instead of wall-clock time.

Cheers,
Patrick

On Tue, Oct 12, 2010 at 11:41 PM, Peshala Jayasekara
<peshala_24@yahoo.com>wrote:

> Hi all,
>
> I want to know if there is a way to change the time stamp of a bagfile when
> you play it.
> For example, when you run robot_pose_ekf of navigation stack, it compares
> current time with sensor time stamps and it will not update the filter for
> old sensor data.
>
> Thank you in advance.
> Peshala
>
>
>
>
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