How to export time data (to .txt file) published on /clock topic by playing a rosbag file ?

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I have a rosbag file which publishes time values to /clock topic when I play it.

I need to save these time values to a .txt file.

There is one relevant solution as shown below:

rostopic echo -b file.bag -p /topic > data.txt

However, in my case the /clock topic publishes data of type ros::Time

Therefore, data cannot be written to text file. I need these ros time values in double sec; so that it can be written to .txt file.

Any suitable solution for this problem ?