a 20-second uncertainty in realizing AMT. In order to refine the location of the prime meridian, it has been proposed that it be based on a specification that the Viking Lander 1 is located at 47.95137°W. Lander mission clocks When a NASA spacecraft lander begins operations on Mars, the passing Martian days (sols) are tracked using a simple numerical count. The two Viking mission landers, Mars Phoenix, the Mars Science Laboratory rover Curiosity, InSight, and Mars 2020 Perseverance missions all count the sol on which the lander touched down as "Sol 0". Mars Pathfinder and the two Mars Exploration Rovers instead defined touchdown as "Sol 1". Each successful lander mission so far has used its own "time zone", corresponding to some defined version of local solar time at the landing site location. Of the nine successful NASA Mars landers to date, eight employed offsets from local mean solar time (LMST) for the lander site while the ninth (Mars Pathfinder) used local true solar time (LTST). Information as to whether China's Zhurong rover project has used a similar timekeeping system of recording the sol number and LMST (or offset) has not been disseminated. Viking Landers The "local lander time" for the two Viking mission landers were offsets from LMST at the respective lander sites. In both cases, the initial clock midnight was set to match local true midnight immediately preceding touchdown. Pathfinder Mars Pathfinder used the local apparent solar time at its location of landing. Its time zone was AAT-02:13:01, where "AAT" is Airy Apparent Time, meaning apparent (true) solar time at Airy-0. The difference between the true and mean solar time (AMT and AAT) is the Martian equation of time. Pathfinder kept track of the days with a sol count starting on Sol 1 (corresponding to MSD 43905), on which it landed at night at 02:56:55 (mission clock; 4:41 AMT). Spirit and Opportunity The two Mars Exploration Rovers did not use mission clocks matched to the LMST of their landing points. For mission planning purposes, they instead defined a time scale that would approximately match the clock to the apparent solar time about halfway through the nominal 90-sol primary mission. This was referred to in mission planning as "Hybrid Local Solar Time" (HLST) or as the "MER Continuous Time Algorithm". These time scales were uniform in the sense of mean solar time (i.e., they approximate the mean time of some