What is time?
Time helps us describe when things happen, put events in order, and measure how long something lasts. Clock numbers are one way we keep track of it.
An event, a duration, a clock reading
- An event
- You start reading a story. Later, you finish it. Time lets you describe their order.
- A duration
- You read for ten minutes. That is how much time passed between the two events.
- A clock reading
- You started at 4:00. The number is a label: you also need a date and place or time zone to identify the moment.
Clocks measure intervals by counting something that repeats regularly, such as a pendulum’s swing or an electronic signal. We agree on units so we can compare measurements.
A little deeper: what is a second?
The scientific unit of time is the second. Its definition uses a particular transition in caesium-133 atoms: 9,192,631,770 periods of the corresponding radiation make one second. This gives laboratories a reproducible reference. It does not mean an atom is a tiny clock with hands.
Why do places have different times?
As Earth turns, different places face the Sun at different times. Solar noon happens when the Sun crosses a place’s local north–south line in the sky. Places farther east reach solar noon before places farther west.
Many towns once set clocks by their own local solar time. Time zones let a whole region share clock readings instead. The starting idea was about 15° of longitude for each hour, but actual zones follow human decisions as well as geography.
How GMT and time zones took hold
Greenwich Mean Time (GMT) grew from mean solar time at the Royal Observatory in Greenwich, London. “Mean” means average: the Sun’s apparent progress is not perfectly even through the year. Averaging it made a steadier basis for clocks; noon GMT need not be the exact moment of solar noon.
- Late 1600s: Greenwich astronomers used accurate pendulum clocks and worked with local mean time.
- 1700s: Greenwich observations, navigation tables and marine timekeepers helped sailors find longitude.
- 1800s: Railways made different town times confusing. British railway companies adopted GMT; the Railway Clearing House agreed to it in 1847. Telegraph wires then helped distribute the time.
- 1883: U.S. railways introduced a coordinated standard-time system. Adoption of standard time was a process, not one worldwide switch.
- 1884: The International Meridian Conference in Washington, D.C. chose Greenwich as the common starting meridian for longitude.
Read the histories from Royal Museums Greenwich and NIST.
Why is Greenwich longitude zero?
A globe has no natural starting line for longitude. Countries had used different reference meridians. In 1884, delegates chose Greenwich because many ships and navigators already used charts based on it, making a shared reference less disruptive.
It was an international historical agreement, not a special physical property of Greenwich. The prime meridian tells us where we agreed to start measuring east and west.
Does today’s GPS zero run through the old telescope?
Not exactly. The historic line was defined by Greenwich’s Airy Transit Circle telescope. Modern satellite mapping uses an Earth-centered reference, whose zero line passes about 102 meters east of that historic marker. This does not change the reason Greenwich became the reference.
GMT and UTC today
Coordinated Universal Time (UTC) is the modern international reference for civil timekeeping. It is calculated using atomic-clock data from laboratories around the world. It is not produced by one master clock in Greenwich.
In everyday civil use, GMT means UTC+00:00, so both give the same clock reading. Technically, the original astronomical GMT came from Earth’s rotation, while UTC is an atomic time scale. They have different definitions.
A little deeper: Earth’s rotation and atomic clocks
UT1 is the modern time scale that tracks Earth’s rotation. UTC is based on atomic time and has used leap-second adjustments to keep it close to rotation-based time. For precise work, use the named time scale you mean rather than treating GMT, UT1 and UTC as interchangeable.
BIPM: international time scales · NIST: how UTC and GMT relate
The UK uses GMT in winter and British Summer Time (BST), UTC+01:00, during its summer-time period. UTC itself does not move forward for summer. See the UK government’s clock-change guide. Or learn why clocks change and what standard and daylight time mean.
Leap years and leap seconds: two different fixes
A leap year adds a calendar day. Earth’s journey around the Sun takes a little longer than 365 days. Adding February 29 in selected years helps the calendar stay aligned with the seasons.
For the Gregorian calendar, a year divisible by 4 is usually a leap year. Century years must also be divisible by 400. So 2024 and 2000 are leap years; 1900 and 2100 are not. A leap year has 366 days.
US Naval Observatory: the Gregorian leap-year rule
A leap second adjusts a clock time scale. Earth’s rotation is slightly uneven. UTC has used occasional leap-second adjustments to stay close to rotation-based time. During a positive leap second, a suitable UTC clock can show 23:59:59 → 23:59:60 → 00:00:00.
Leap seconds do not follow the four-year leap-year rule, and they are unrelated to daylight saving. Future adjustments depend on official announcements and the applicable international timekeeping policy.
NIST: leap-second questions · IERS: official timekeeping bulletins
What does that mean for these calculators?
Date calculations include valid February 29 dates. The age calculator lets you choose February 28 or March 1 for leap-day birthdays in other years; that is a calculation preference, not a statement of legal birthday rules.
The Unix timestamp tool and JavaScript clocks do not represent a separate 23:59:60 leap second. Their elapsed seconds are ordinary computer timestamps, not precise atomic-time measurements across leap-second events.
Try the age calculator · Try Unix timestamps · NTP: leap seconds and computer time scales
Real time zones are more than neat slices
Governments choose time-zone boundaries and daylight-saving rules. Offsets are not always whole hours: some include 30 or 45 minutes. Borders bend, rules can change, and solar noon often differs from 12:00 on a wall clock.
The International Date Line is roughly on the other side of Earth from Greenwich, near 180° longitude. It bends around political boundaries. Crossing it changes the local calendar date; it does not make you a day older or younger.
NIST: who decides time-zone rules · NOAA: the International Date Line
For a meeting or journey, choose a named place and a date. A date-aware converter can use that place’s rules for that moment.
Try it for yourself
- Compare dates and times around the world
- Move the hands in Clock Playground
- Explore how motion affects measured time in Relativity Lab
Time zones change the labels used for everyday coordination. Relativity asks a different question about how physical clocks compare.
Sources reviewed . This introduction uses everyday examples; it is not a complete theory of time.