Einstein showed that clocks don't all tick at the same rate. How fast you move and how deep you sit in a gravity well both change it. Play with both effects below.
Special relativity: the faster you move relative to Earth, the slower your clock runs compared to Earth's.
General relativity: the deeper you sit in a gravity well, the slower your clock runs compared to a clock far from any mass.
v = your speed, c = speed of light. If you experience ฯ years of travel at speed v, Earth experiences ฯยทฮณ years. This ignores the acceleration needed to turn around and come home, which adds further asymmetry in the classic "twin paradox."G = gravitational constant, M = mass of the body, r = your distance from its center, c = speed of light. This is the Schwarzschild solution for a non-rotating mass; 2GM/cยฒ is the Schwarzschild radius, the event horizon of a black hole of mass M.