Einstein's General Theory of Relativity shows that time is a physical thing. It is part of space-tme. Space-time manifests itself as a gravitational field created by the mass of objects. The presence of mass will warp, or deform, space-time. As a consequence, time passes more slowly near to a massive object such as a planet. This has been demonstrated to be true by comparing super-accurate clocks in orbit to clocks on the surface of the Earth. The clocks on the surface run just a little bit slower. (This is a paraphrase from other people's explanations)