I have the same question from a diferent perspective. The more matter you have around you/the deeper you are in a gravity well (same thing), the slower time runs. This question asks about time on planets; I want to ask about time in the center of megagalactic voids. Time runs faster there than it does inside a cosmic thread of galaxies. So when I see a claim that the universe is 13.8 billion years old, I wonder how old it is depending on where you are. Has anyone seen a contour map of what time it is in the universe?