Coordinated Universal Time (UTC) is the basis for modern civil time. Since January 1, 1972, it has been
defined to follow International Atomic Time (TAI) with an exact offset of an integer number of seconds,
changing only when a leap second is added to keep clock time synchronized with the rotation of the Earth.

Greenwich Mean Time (GMT) is an older standard, adopted starting with British railroads in 1847. Using
telescopes instead of atomic clocks, GMT was calibrated to the mean solar time at the Royal Observatory,
Greenwich in the UK. Universal Time (UT) is the modern term for the international telescope-based system,
adopted to replace "Greenwich Mean Time" in 1928 by the International Astronomical Union. Observations at
the Greenwich Observatory itself ceased in 1954, though the location is still used as the basis for the
coordinate system. Because the rotational period of Earth is not perfectly constant, the duration of a second
would vary if calibrated to a telescope-based standard like GMT or UT - in which a second was defined as a
fraction of a day or year. The terms "GMT" and "Greenwich Mean Time" are sometimes used informally to
refer to UTC

Time Zones

Earth is split up into a number of time zones. Most time zones are exactly one hour apart, and by convention
compute their local time as an offset from UTC or GMT. In many locations these offsets vary twice yearly due
to daylight saving time transitions.

A time zone is a region on Earth, more or less bounded by lines of longitude, that has a uniform, legally
mandated standard time, usually referred to as the local time. By convention, the 24 main time zones on Earth
compute their local time as an offset from UTC, each time zone boundary being ostensibly 15 degrees east or
west of the preceding one. The reference point for UTC is the Greenwich Meridian (the Prime Meridian),
which has a longitude of 0°. Local time is UTC plus the current time zone offset for the location in question. A
corresponding one hour decrease relative to UTC occurs every 15° heading westward from the western
boundary of the UTC time zone, up to the International Date Line

Daylight Saving Time

Daylight Saving Time (DST) also known as "summer time" is the practice of temporarily advancing clocks so
that afternoons have more daylight and mornings have less. Typically clocks are adjusted forward one hour
near the start of spring and are adjusted backward in autumn.

In a typical case where a one-hour shift occurs at 02:00 local time, in spring the clock jumps forward from 02:
00 standard time to 03:00 DST and that day has 23 hours, whereas in autumn the clock jumps backward from
02:00 DST to 01:00 standard time, repeating that hour, and that day has 25 hours. A digital display of local
time does not read 02:00 exactly at the shift, but instead jumps from 01:59:59.9 either forward to 03:00:00.0
or backward to 01:00:00.0. In this example, a location observing UTC+2 during standard time is at UTC+3
during DST; conversely, a location at UTC−5 during standard time is at UTC−4 during DST. Clock shifts are
usually scheduled near a weekend midnight to lessen disruption to weekday schedules.

Space Time

In space, light and time act differently than here on Earth, and speed is a different concept when time can
theoretically be stopped in its tracks.

The speed of light is 186,000 miles per second, or approximately 670 million miles per hour. Nothing in the
universe travels faster than light: at the speed of light, time stops. No matter how fast you are moving when
you observe it, the speed of light will always be 186,000 miles per second. In physics, the three dimensions of
space and the fourth, separate dimension of time combine to form a single construct throughout the cosmos
called space-time. Using space-time, scientists can measure both the place and time something occurs in one.

Time is relative to the person observing it and time may change depending on how fast the observer is
moving. Time passes more slowly for satellites in orbit when compared to time on Earth because those
satellites are moving. Satellites used for the Global Positioning System are launched with their clocks running
faster so that when they reach orbit they mark time the same as Earth-based clocks. Because time passes
more slowly in orbit around Earth, astronauts arrive home a little younger than if they had spent the same
amount of time on Earth. If you were to travel near the speed of light for what seems to be 20 years, you
would return to Earth thousands of years in the future