The Problem with Local Time in Space
The International Space Station (ISS) orbits Earth at approximately 28,000 km/h, completing one orbit every 90 minutes. In a 24-hour period, crew members experience 16 sunrises and 16 sunsets. Using local solar time is obviously impossible — the sun rises every hour and a half.
The ISS Uses UTC
The ISS officially keeps UTC (Coordinated Universal Time) as its onboard time. This was chosen because both major partners — NASA (American crews) and Roscosmos (Russian crews) — need a neutral common reference. UTC is also consistent with mission control centers in Houston (UTC−5/−6) and Moscow (UTC+3), both of which must coordinate in real time.
Crew Sleep Schedules
Astronaut work and sleep schedules are planned around UTC, not local sunrise/sunset. The station's windows are fitted with blackout shades, and artificial lighting cycles simulate a roughly 24-hour day. Sleep periods are typically around midnight UTC. Because the ISS experiences 16 day/night cycles per day, the crew's circadian rhythm is managed entirely through lighting, scheduled meals, and exercise — not natural light.
Mars Time — A New Problem
Mars has a day (a "sol") of 24 hours, 39 minutes, and 35 seconds — about 39 minutes longer than an Earth day. NASA's Mars rovers operate on Mars solar time, and mission controllers who work rover schedules must live on "Mars time," which shifts 39 minutes later each Earth day. During the Mars Pathfinder mission in 1997, controllers wore special watches that ran on Mars time to keep their schedules synchronized.
Future Space Habitats
As humanity plans for Moon and Mars bases, the question of time zones in space becomes more practical. A lunar base would experience about 14 Earth days of continuous sunlight followed by 14 days of darkness — local solar time is essentially useless. Proposed solutions include using UTC, Earth time zones from the home country of the base operator, or a custom "lunar time standard."
Deep Space Missions
Interplanetary spacecraft like the Voyager probes communicate using timestamps referenced to UTC. As light-travel times grow (the Voyager 1 signal takes about 22 hours one-way), the concept of "simultaneous" becomes blurry and engineers must account for signal delay in all timing calculations.