This book is unique and exceptional in dealing with the notion of physical time rigorously, both logically and empirically. The central theme is the intimate relation between physical time and cosmic gravity. It establishes and explains, in an accessible manner, the one crucial physical fact that has been missed in the development of modern physics-that the enormous gravity of the matter and energy in the Universe is the controller and cause of the relativistic time. The material in the book is accurate and free of the ambiguities in the discussion of time and its modifications (dilation), synchronization of clocks, and simultaneity. The contents go beyond the current theories of relativity that fail to incorporate the cosmic gravity in their structure. The discussion of clocks in satellite navigational systems (like the GPS) is the most complete and accurate. The book offers several new insights, and it is the only available treatise on the complete physical truth about time. The contents are addressed to a wide range of readers, from general readers and students to experienced researchers, and will also appeal well to philosophers and historians of physics. This book has the enabling quality to deal with difficult questions about physical time, with unprecedented clarity and without paradoxes.