Time may seem like one of the most predictable, dependable measures we’ve got. But despite the reliable tick, tick, tick of your wristwatch, time doesn’t always flow at the same rate. Einstein’s theories say that time varies depending on things like gravity and acceleration, so that time moves more slowly the further into an object’s gravity you go. That means that the center of the Earth must be younger than its surface. According to a 2016 study, it is — by more than we ever believed.