OK, so it's about gravitational time dilation.
I'm soft on the specifics of General Relativity.
A plot of (Newtonian) gravitational acceleration (g) by distance from the center of mass (r) looks like this:
Where the planet has radius R, and g is the acceleration of gravity at a given distance from the center.
So I'd assume that the surface of the Earth has the highest gravity (most dramatic time dilation), since it is decreasing as you move either toward or away from the center.
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