Time is not an illusion and it has nothing to do with gravity, it is constant throughout the universe. However, humans perception of time is based on the Earths' rotation around the orbit of the Sun, which is irrelevent to the universe's perception of time. What you have to do is find a constant throughout the universe, such as the speed of light, or the half-life decay of elements, or the pulse rate of quasars, etc. Then create a universal time standard based on that. But time is not an illusion! It would take you the same amount of time to take a shit out at the edge of the universe as it does here!!!