How to read the actual date in ROS

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Hey!, I would like to name some archives, which I create in a ROS node, with a string related to the actual date on which the node is called. I already have used Ros time, but I dont know what does this "absolute" time means. My code is:

ros::Time begin = ros::Time::now();
ROS_INFO("%d\n", begin.sec);

And the output is:

[ INFO] [1508853591.867555047, 1508853591.867143646]: 1508853591

My questions are, what does this "1508853591" means? And how could I get something like 24-10-2017-16:13 or like it?

Thanks!