Time is a concept to explain change. Since gravity is a force that causes change, time can be defined using the rate at which gravity is propagated. (gravity-time). The speed of gravity is invariant in the pan-inertial frame and is not affected by other gravity, gravity-time is homogeneous even in the gravitational field. (In an inertial frame, gravity-time is equal to the time of the special theory of relativity). Gravity, created by a point mass, can be removed by using a reference frame of uniform circular motion (speed v) so that the speed of light in the gravitational field can be calculated. Here, if Newtonian mechanics is applied as an approximation and the above equation is converted into an equation of radius R, then the speed of light and the size of the black hole exactly match the values calculated by the general theory of relativity. The principle of constancy of the speed of gravity establishes the Lorentz transformation. When viewed from a moving object in respect to a star, gravity-time in the star passes slowly. Due to the difference in the time flow between the star and the object, the gravity transmitted from the star for a certain time reaches the moving object in a longer time, so the gravity works weakly compared to a stationary object. The gravity-time equations of motion could be represented using the momentarily co-moving pan-inertial frame. With this equation, we can show that even inside a black hole, the acceleration or the acting force does not become infinite. Also, the size of a black hole is smaller than the Schwarzschild radius. The theory of gravity-time predicts that gravitational wave is faster than light in a gravitational field, and that gravity works weakly on a moving object compared to a stationary one.