One hundred years ago Einstein's Relativity Theory changed our understanding of time. Rather than a single universal clock, ticking the same way everywhere, he realized that time depends on the motion of the clock. What's more, he saw that gravity is nothing but the warping of time (and space), so that time depends also on the location of the clock. Relativity theory has now been observed in many phenomena, and is both a practical matter and a means of understanding and exploring the cosmos, for instance through its predictions of bending of light rays, black holes, expansion of the universe, and gravitational waves: ripples of time and space. But the theory presents yet deeper mysteries which it is unable to address.