In general relativity, time dilation occurs in the presence of gravitational fields. According to this theory, time runs slower in regions with stronger gravitational fields. Therefore, the closer you are to a massive object, such as the center of a planet, the more pronounced the time dilation effect becomes compared to regions with weaker gravitational fields, such as the planet's surface.
To understand this concept, let's consider a thought experiment. Suppose you have two observers, one located at the center of a planet and another at the planet's surface. The observer at the center experiences a stronger gravitational field because they are closer to the planet's mass. As a result, time will run slower for this observer relative to the observer at the planet's surface.
This effect can be attributed to the curvature of spacetime caused by the planet's mass. The closer an observer is to the center, the deeper they are within the gravitational well, and the stronger the time dilation effect becomes.
It is worth noting that the time dilation effect near the surface of a planet is still present but relatively weaker compared to the center. This difference is more pronounced near extremely massive objects like black holes, where the time dilation effect can be significantly significant near the event horizon. However, for typical planets, the time dilation effect is relatively small and not easily noticeable in everyday life.