Timekeeping beyond Earth means adapting clocks and calendars to places where the Sun rises more than 16 times a day or where a day lasts 39 minutes longer than on Earth. Astronauts on the International Space Station (ISS) follow UTC, while Mars missions schedule by sols—Martian days. This guide explains how space agencies coordinate time off-world, surveys proposed Martian calendars, and looks ahead to future space observances and standards.

What does “timekeeping beyond Earth” mean?

On Earth we anchor daily life to a 24-hour cycle and annual seasons defined by our planet’s orbit. Beyond Earth, neither the Sun’s apparent motion nor Earth’s calendar neatly applies. Space operations rely on:

• Stable, universal references (e.g., UTC, atomic time, Julian dates)
• Local solar cycles and “days” on other worlds (e.g., the Martian sol)
• Mission-specific clocks (e.g., Mission Elapsed Time)
• Relativistic corrections and light-time for navigation and communication

How astronauts keep time on the ISS

The ISS circles Earth about every 90–92 minutes, giving the crew roughly 16 sunrises and sunsets per Earth day. Yet their watches don’t reset with each orbit.

The baseline: UTC, TAI, and leap seconds

Coordinated Universal Time (UTC) is the station’s official time. Mission control centers around the world plan crew schedules, experiments, and vehicle rendezvous on UTC so everyone shares a single reference. UTC itself is tied to a network of atomic clocks and occasionally adjusted with leap seconds to stay aligned with Earth’s rotation.

Two related time scales are common behind the scenes:

• TAI (International Atomic Time): a continuous atomic timescale without leap seconds, useful for precise engineering timelines.
• UT1: a measure of Earth’s actual rotation, used for pointing antennas and telescopes.

Relativity and orbital day–night cycles

At ISS altitude (~400 km), special and general relativity slightly affect clocks. Because the station moves fast (~7.66 km/s), time aboard runs a little slower than on Earth’s surface due to special relativity, while being farther from Earth’s mass makes clocks run slightly faster due to gravity. The net effect is tiny—on the order of a few tens of microseconds per day—so daily schedules simply use UTC. Navigation systems (like GPS) already incorporate relativity so astronauts don’t have to.

Despite frequent sunrises, the crew keeps a normal human day:

• Fixed sleep/wake schedule: lighting cues and window shutters simulate a 24-hour circadian rhythm.
• Planned “day”: work, exercise, and meals occur at specific UTC times.
• Event timing: dockings, EVAs, and burns are anchored to UTC down to seconds.

Scheduling humans, not sunsets

For astronauts, timekeeping is about team coordination and health, not matching the external light cycle. Mission planners prioritize predictable routines, reduced circadian disruption, and global coordination across Houston, Tsukuba, Baikonur, Cologne, and other centers—all on UTC.

Spacecraft and deep-space timekeeping

Uncrewed spacecraft don’t sleep; they keep time for navigation, sequencing, and communication.

Mission Elapsed Time, epochs, and Julian dates

• Mission Elapsed Time (MET): counts from launch or another defined “zero” (e.g., MET 12:03:15 since liftoff).
• Julian Date (JD) and Modified Julian Date (MJD): continuous day counts used in astronomy and operations to avoid calendar complications.
• Command sequencing: onboard computers execute time-tagged events relative to MET or absolute time scales.

Atomic clocks, DSN, and light-time

For deep-space missions, timing accuracy directly supports navigation:

• Deep Space Network (DSN) tracking uses precise two-way radio timing to determine spacecraft range and velocity.
• Atomic clocks aboard or on the ground provide the stability needed for navigation solutions measured in billionths of a second.
• Light-time delay: signals take minutes to reach Mars (and hours for the outer planets). Every command is time-tagged to account for current light-time.
• Relativistic coordinate times (e.g., TDB/TCB) are used in trajectory and ephemeris software so that planetary motions and signal travel times are modeled consistently in the solar system’s barycentric frame.

Why Mars missions use sols

A sol is a Martian solar day—the interval between two noons at a fixed location on Mars. It is the natural unit for scheduling surface operations.

How long is a sol?

• Length: about 24 hours, 39 minutes, 35 seconds (≈1.0275 Earth days).
• Martian year: about 668.6 sols (687 Earth days).

Because a sol is slightly longer than an Earth day, keeping Mars rovers on “Earth time” would slowly desynchronize activities from daylight at their site. Using sols ensures activities—driving, sampling, imaging—line up with local sunshine and temperatures.

Local solar time on Mars: LMST, LTST, MSD, and MTC

Mars operations often reference local solar times:

• LMST (Local Mean Solar Time): local time averaged over the Martian year; similar to civil time zones on Earth.
• LTST (Local True Solar Time): local time based on the actual Sun position; it varies by the “equation of time.”
• MSD (Mars Sol Date): a continuous count of sols (fractional) since a defined epoch, analogous to Julian Date.
• MTC (Mars Coordinated Time): a proposed Mars-wide standard analogous to UTC, centered on the Airy-0 meridian; not an official civil time, but used in research contexts.

Rover teams combine these with geography. For instance, “Sol 178 at 11:30 LMST, Gale Crater” uniquely identifies a time that corresponds to late morning at Curiosity’s site.

Living on “Mars time” on Earth

During the first months of surface missions, Earth-based teams often adopt shift schedules synchronized to the rover’s local solar day. That means daily start times drift ~39 minutes later each Earth day. Teams for Spirit, Opportunity, Curiosity, and Perseverance have used wearable light therapy and blackout curtains to cope. After early mission phases, operations typically transition to Earth day shifts while maintaining sol-based planning.

Proposed Martian calendars

When people imagine settlements on Mars, a calendar soon follows. A civil calendar must balance human expectations (weeks, months, holidays) with Martian realities (668.6-sol year, noticeable seasonal differences between hemispheres, and a 24h:39m day).

The Darian calendar (overview)

The Darian calendar is the best-known proposal tailored to Mars. Its goals: align month starts with weekdays predictably, keep seasons reasonably stable, and use familiar structures adapted to sols.

• Months: 24 months per Martian year, typically alternating lengths so weeks and months line up neatly.
• Weeks: 7-sol weeks retained for cultural continuity.
• Intercalation: extra sols distributed across a multi-year cycle to keep the calendar close to the Martian tropical year (668.6 sols), analogous to leap days.
• Seasonal anchoring: month layout attempts to keep northern hemisphere seasons in roughly the same months across years.

The key challenge is the fractional year length. Just as Earth uses a leap-day system to handle 365.2422 days per year, a Martian calendar needs a leap-sol scheme to approximate 668.5921 sols per Martian year without drifting too fast relative to the seasons.

Other ideas: fixed weeks, decimal time, dual clocks

Beyond Darian, designers have explored a variety of approaches:

• Fixed-week calendars: Every month has a whole number of weeks (e.g., 28-sol months), with intercalary festival days outside the week cycle to preserve weekday consistency.
• Decimal time: Divide the sol into 10 “decimal hours,” 100 decimal minutes, etc. It simplifies arithmetic but breaks with Earth norms and introduces conversion friction for interplanetary coordination.
• Dual clocks: Daily life runs on local sol time; interplanetary business uses a universal standard (UTC or a Mars-wide MTC). Devices display both at once.
• Season-first calendars: Define months by Areocentric Solar Longitude (Ls)—the Mars-specific measure of season—so month boundaries lock to equinoxes and solstices. Month lengths then vary.
• Epoch choices: Some proposals anchor Year 1 to a culturally significant future Mars milestone (first human landing), while scientific practice often uses defined epochs tied to observations.

What would adoption require?

A Martian civil calendar would need:

• Legal definition by the governing authority of a settlement or consortium.
• Standards for leap-sol rules, week structure, and month names, published for manufacturers and software developers.
• Interoperability with Earth systems—clear mapping between UTC/TAI and local sol time to avoid safety and financial errors.
• Public buy-in: Schools, workplaces, and media all aligned to the same calendar for everyday use.

Calendars for a multi-planet future

As humans operate across Earth orbit, the Moon, Mars, and beyond, we’ll need layered time standards.

Coordinated Planetary Time?

Think of a stack of complementary clocks:

• Global coordination layer: UTC remains the backbone for multi-agency, multi-world synchronization. For science and navigation, atomic and relativistic time scales (TAI, TDB/TCB) underpin computation.
• Local civil layers: Each world adopts a civil time tied to its solar day and seasons (e.g., a lunar time for the Moon, MTC/LMST-based civil time for Mars).
• Conversion layer: Standardized algorithms and metadata let devices reliably translate between UTC and local times, including leap seconds or leap sols.

Policy momentum is already visible: international partners have discussed standard time for lunar operations, and work toward defining a consistent lunar timescale is underway to support navigation networks and surface coordination. Mars will likely follow once sustained human operations begin.

Future space holidays and observances

Calendars are also culture. Expect new traditions alongside technical standards:

• Founding Days: anniversaries of first landings, base foundings, or first harvests in controlled habitats.
• Synodic Festivals: recurring every ~780 Earth days when Earth and Mars align favorably for travel.
• Perihelion/Aphelion Days: marking orbital milestones unique to each world.
• Transit Day: commemorating the start of an interplanetary journey.
• Leap-sol Festivals: cultural events tied to the insertion of intercalary sols in a Martian calendar.

Key takeaways

• ISS time: Astronauts live by UTC for health and coordination, ignoring frequent orbital sunrises.
• Spacecraft time: Missions rely on MET, Julian dates, and atomic-clock-backed networks with relativistic and light-time corrections.
• Mars operations: Use sol-based planning and local solar time (LMST/LTST) so work lines up with daylight.
• Martian calendars: Proposals like the Darian calendar handle 668.6-sol years with leap-sol rules, but no civil standard exists yet.
• Future standards: A layered approach—UTC for coordination, local times for daily life—will likely define calendars for space, the Moon, and Mars.

FAQ

How do astronauts tell time on the ISS?

They use Coordinated Universal Time (UTC). Daily timelines, vehicle rendezvous, and science tasks are all planned in UTC, with lighting and schedules set to maintain a healthy 24-hour circadian rhythm despite ~16 sunrises per day.

Why do Mars missions use sols instead of Earth days?

A sol—the Martian solar day—is about 24 hours 39 minutes. Planning in sols keeps rover activities aligned with local daylight and temperature, which is essential for power, imaging, and safe driving.

Does relativity affect timekeeping in space?

Yes, but its effects on human schedules in low Earth orbit are tiny—tens of microseconds per day on the ISS. Navigation systems and atomic-clock standards account for relativity so operations and communications remain synchronized.

Is there an official Martian calendar?

No. Scientists use tools like Mars Sol Date (MSD) and Local Mean Solar Time (LMST) for operations and research. Civil calendars such as the Darian proposal exist on paper but haven’t been adopted by any Martian settlement (there aren’t any yet).

What is Mars Coordinated Time (MTC)?

MTC is a proposed Mars-wide time standard analogous to UTC, referenced to the Airy-0 meridian. It’s useful in studies and software but isn’t an official civil time or widely used outside technical contexts.

How do spacecraft keep precise time far from Earth?

They use onboard clocks synchronized with ground systems, time-tag commands and data packets, and rely on the Deep Space Network’s precise ranging and Doppler tracking. Timing models include light-time and relativistic corrections to maintain accuracy.

What about time on the Moon?

Lunar missions currently coordinate with Earth time (UTC) and mission-specific clocks. Work is advancing toward a consistent lunar timescale to support navigation and surface operations as activity on and around the Moon expands.