What exactly does read_messages for a bag returns?

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In the documentation for rosbag it referes to the function bag.read_messages which returns three values:

topic: the topic of the message

msg: the message

t: time of message. The time is represented as a rospy Time object (t.secs, t.nsecs)

I have a rosbag with messages and these messages do have timestamps so these timestamps are inside the msg.header.stamp field.

My question is, what exactly is t here (the "time of message") because I have experimented it and it is definitely not the timestamp of the message.