Time keeping on two sides of another planet turns out to be a challenge, at least when calibrated from our own rhythms. On Earth, it took thousands of years for navigators to get terrestrial time on some reliable global standards. But when not on our globe, the familiar clock ticks need a sun calibration. Some of the first interplanetary clocks started during the 1976 Viking Mars mission, but since then interplanetary time-keeping has moved on. NASA’s Michael Allison describes how to wind a Martian watch.