The mineral zircon is the oldest known piece of Earth existing on the surface today. The oldest bits date back as far as 4.37 billion years — not too far from the age of Earth itself at about 4.5 billion years old. And, unlike other minerals, zircon is hard to get rid of. This resilience enables scientists to use zircon to determine when major geological events on Earth happened. As part of our series on time, host Aaron Scott talks to science correspondent Nell Greenfieldboyce about why this mineral is often considered a geologic clock and has earned the nickname "Time Lord." This episode is part of our series, "Finding Time — a journey through the fourth dimension to learn what makes us tick." Read more of Nell's reporting on zircon here. Curious about other aspects of our universe? Email us at ShortWave@NPR.org.