Time travel might exist to the future, but never backward, if not where are the tourists from the future !! as Hawking mentioned in the book with a humorous manner..

This book is definitely not a waste of time, a briefer version to "A brief history of time" (and newer one) this book is a distilled exploration of the cosmos, presenting complex scientific concepts in an accessible and engaging manner.
Like science, this book might be informative today but inaccurate tomorrow, Hawking reminds us that science is not static but a dynamic process, continually refined and reshaped by new discoveries. Many theories and informations might evolve or radically change or might be replaced altogether so a new book of that kind might appear every now and then, but it has its advantages and it helps us to understand few things better and clearer, and like the universe that evolves , the science must follow and so literature.. And so our understanding of phenomena like gravity, quantum mechanics, and the electromagnetic forces governing atoms and planets is in constant flux.

To conclude in few words, Hawking encourages us to embrace the uncertainties of scientific knowledge because this reflects the beauty of science as an ongoing journey and not a final destination..