On Earth, we divide the planet into time zones based on longitude and politics. It is a messy but functional system that has served us since the 1880s. But what happens when humans venture beyond Earth? The International Space Station orbits at 28,000 km/h, seeing 16 sunrises every 24 hours. The Moon has no political boundaries (yet) and a day-night cycle of about 29.5 Earth days. Mars rotates at almost the same rate as Earth but sits millions of kilometers away with a signal delay of up to 24 minutes. Timekeeping off Earth is a genuinely unsolved problem.
How the ISS keeps time
The International Space Station uses Coordinated Universal Time (UTC) as its onboard standard. This was a pragmatic choice: the ISS is a collaboration between the US, Russia, Europe, Japan, and Canada, and UTC is the internationally agreed-upon reference. Crew schedules, experiments, and communications all follow UTC. The astronauts wake up, eat, exercise, and sleep according to a UTC-based schedule, regardless of whether they are flying over daylight or darkness at any given moment. Mission Control in Houston (UTC-6 or UTC-5 during DST) and TsUP in Moscow (UTC+3) each convert to their local time as needed. The station orbits Earth roughly every 90 minutes, which means the crew experiences about 45 minutes of daylight and 45 minutes of darkness per orbit. The window shutters are programmed to create an artificial day-night cycle that aligns with the UTC schedule, helping the crew maintain a normal circadian rhythm despite the bizarre light conditions.
The lunar time question
As space agencies plan permanent lunar bases (NASA's Artemis program, ESA's Moon Village concept, China's ILRS), the question of lunar timekeeping has become practical, not hypothetical. In November 2022, the European Space Agency published a position paper calling for the establishment of a common lunar reference time. The challenge is that a lunar day (sunrise to sunrise) lasts about 29.5 Earth days, making Earth-style time zones based on the sun's position impractical. A lunar base on the near side might experience about 14.75 days of continuous sunlight followed by 14.75 days of darkness. The leading proposal is to use a version of UTC as the base time on the Moon, possibly with a small correction factor to account for relativistic effects. NASA has already begun drafting a framework called Coordinated Lunar Time (LTC), with a target of having a formal standard in place before crewed Artemis missions establish sustained presence.
Relativity: why clocks tick differently
Einstein's theories of relativity predict that clocks tick at different rates depending on gravity and velocity. A clock on the Moon, which has about one-sixth of Earth's gravity, ticks slightly faster than a clock on Earth's surface, by about 56 microseconds per day. A clock on the ISS, which is moving at high speed but also in lower gravity than the surface, ticks slightly slower due to the velocity effect but slightly faster due to the reduced gravity, with the net result being about 0.01 seconds slower per year. These differences are tiny for daily life but critical for navigation and synchronization. GPS satellites, orbiting at about 20,200 km altitude, have their clocks adjusted to account for both gravitational and velocity effects. Without these corrections, GPS positions would drift by about 10 km per day.
Mars time: the almost-24-hour day
A Martian day (called a "sol") is 24 hours, 39 minutes, and 35 seconds long, tantalizingly close to an Earth day but just different enough to cause problems. During the Mars Exploration Rover missions, NASA teams at the Jet Propulsion Laboratory tried living on "Mars time," shifting their schedules by about 40 minutes each day to stay synchronized with the rovers' operational cycles. After a few weeks, the accumulated drift meant they were working through the night and sleeping during the day. Most team members found it exhausting and disorienting. For future human missions to Mars, the slight mismatch between the Martian sol and Earth circadian rhythms will be a real challenge, possibly requiring purpose-built timekeeping systems that do not map to any Earth time zone.
Toward a solar system time standard
As human activity expands beyond Earth, the need for a common time reference will grow. The current approach of using UTC everywhere works when communication delays are small (the Moon is about 1.3 light-seconds away). For Mars (4 to 24 light-minutes away), real-time coordination is impossible, and local time systems will be necessary. Some proposals suggest a hierarchical system: UTC on Earth, a Lunar Time (LT) for the Moon, and a Mars Time (MT) for the Red Planet, with conversion standards between them. The physics of relativity means that perfectly synchronizing clocks across the solar system is fundamentally impossible, but practical approximations will be developed as the need arises. The first Moon bases will likely just use UTC with a note about the 56-microsecond daily drift. That is good enough for now, though it will not remain sufficient forever as the precision demands of lunar navigation and communication grow.