Is gravity stronger or weaker at the center of the Earth?

I have always thought that gravity was strongest at the surface of a planet. Science fiction often depicts this. However, the June 25, 2016 issue of Science News (page 5) states that “In Einstein’s general theory of relativity, massive objects warp the fabric of spacetime, creating a gravitational pull and slowing time nearby. So a clock at placed at placed at Earth’s its surface will tick ever-so-slightly slower than a clock at its surface. Such time shifts are determined by the gravitation potential, a measure of the amount of work it would take to move an object from one place to another. Since climbing up from Earth’s center would be a struggle against gravity, clocks down deep would run slow relative to surface timepieces.

“Over the roughly 4.5 billion years of Earth’s history, the gradual shaving off of fractions of a second adds up to a core that’s 2.5 years younger than the planets crust, researchers estimate in the may European Journal of Physics.”