When NASA’s Mars Exploration Rover-A, more affectionately known as “Spirit,” touches down in Gusev Crater, it will be approximately 8:30 PM, January 3rd, 2004, at mission control. That’s Pacific Standard Time (PST), because mission control is located on the grounds of the Jet Propulsion Laboratory (JPL), in Pasadena, California. When time is the topic, however, Pacific Standard tells only part of Spirit’s story. Scientists and engineers will also be keeping track of UTC (Coordinated Universal Time), more commonly known as Greenwich Mean Time. UTC is the basis of official timekeeping all over the world. This world, at any rate. Using UTC makes it easier for NASA and its international collaborators to synchronize their watches. So much for Earth. But what about Mars? While international flights on Earth require coordination between two time zones, interplanetary flights require time coordination between two worlds. And coordinating clocks between Earth and Mars is not a simple matter of adding or subtracting hours.