How to associate Rosbag time with data

Hello everyone! I have saved some flight data using rosbag record, but the header of my topic /odom doesn't contain the time. When I extract the data using rostopic echo -b foo.bag -p /odom > odom.csv, the column %time is always 0. I was wondering how can the rosbag and rxbag can synchronize the data... I would like to know the mechanic and how I can get the rosbag time to associate it with the data.