Tracking time is one of those things that seems easy, until you really start to get into the details of what time actually is. We define a second as 9,192,631,770 oscillations of a cesium atom. However, according to Einsteins theory of general relativity, mass slows down these oscillations, making time appear to move more slowly for objects in large gravity wells. This distinction becomes critical as we start considering how to keep track of time between two separate gravity wells of varying strengths, such as on the Earth and the Moon. A new paper by Pascale Defraigne at the Royal Observatory of Belgium and her co-authors discusses some potential frameworks for solving that problem and settles on using the new Lunar Coordinate Time (TCL) suggested by the International Astronomical Union (IAU).