This book is unique and exceptional in any recent literature that deals 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 and systematic 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 entire controller and cause of the relativistic flow of time. The material in the book is accurate and free of the ambiguities and imprecision that is common 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 is the most complete and accurate. The book offers several new insights and results about physical time, 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 to experienced researchers in physics, metrology, and precision chronometry. The content appeals well to philosophers and historians of physics as well. This book has the enabling quality, to deal with difficult questions about physical time, with unprecedented clarity and without paradoxes.