TimeStamp of each topic

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ROS topics themselves do not have timestamps. Each message on a topic may have a timestamp (if it has a header). Even then the messages are published by one node (maybe with a timestamp), but only a subscribing node knows when it receives the message. Depending on what you're trying to do, there may be ways to get the information you want though. For instance, the subscribing node can log when it receives each message as well as the timestamp from the message header to show when it was sent and received. If you want to use the information online, how do you envision using it?