Creating tf from map to base_link

I am using gmapping and robot_pose_ekf. gmapping provides tf from map to odom_combined, and robot_pose_ekf provides odom_combined to base_link. I want to see in rviz the location of the robot which is map to base_link. I therefore wrote a tf publisher to broadcast this transform.

#include 
#include 
#include 

int main(int argc, char** argv){
    ros::init(argc, argv, "tf_linker");
    ros::NodeHandle node;
    tf::TransformListener listener;
    static tf::TransformBroadcaster br;
    ros::Rate rate(20.0);
    while (node.ok()){
        tf::StampedTransform t_bo;
        tf::StampedTransform t_om;
        try{
            listener.lookupTransform("base_link","odom_combined",ros::Time(0),t_bo);
            listener.lookupTransform("odom_combined","map",ros::Time::Time(0),t_om);
        }
        catch(tf::TransformException &ex){
            ROS_ERROR("%s", ex.what());
            ros::Duration(1.0).sleep();
            continue;
        }
        tf::Transform transform;
        transform.setOrigin(t_bo.getOrigin()+t_om.getOrigin());
        transform.setRotation(t_om.getRotation()+t_bo.getRotation());
        br.sendTransform(tf::StampedTransform(transform,ros::Time::now(),"map","base_link"));
    }
}

The problem is that the tf times seems to not be synced by fraction of a second. Here is the error message:

    [ERROR] [1458961860.893869464]: Lookup would require extrapolation into the future.  Requested time 1458961860.893782533 but the latest data is at time 1458961860.893781547, when looking up transform from frame [map] to frame [odom_combined]
Warning: TF_OLD_DATA ignoring data from the past for frame base_link at time 1.45896e+09 according to authority unknown_publisher

Does tf require exactly the same time? That seems very unreasonable. How do I fix it? thanks