It is thought that being close to a gravitational field has a biological effect on all matter including cells. The effect is to slow the movement of electrons in the orbits of their nucleus which thus slows the mechanical and bilological functions of the observer close to the gravity.
Thus the passing of time is a local phenomenon depending on how close you are to a gravitational source.
This is one example of a theory involving “time shells” progressing in size and intensity around a gravitational point from all matter. The more massive the object, the larger and more influencial the time shells around it (like an onion). Another offshoot of this theory is that kinetic energy is actually the conversion of stored energy in the atom as it passes through time shells in a gravitational field.