Time Zones

A time zone is a region that observes a uniform standard time for legal, commercial, and social purposes. Time zones tend to follow the boundaries of countries and their subdivisions because it is convenient for areas in close commercial or other communication to keep the same time.

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Most of the time zones on land are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC−12 to UTC+14), but some governments make local time zones decisions that deviate from the norm. Note that India is +5½ from UTC, while Myanmar (Burma) is +6½, Iran is +4½, Iraq is +3½, Nepal is +5¾ and Central Australia is +9½. Venezuela is -4 1/2, the Canadian island of Newfoundland is -3½ hours from UTC, some smaller islands in French Polynesia are -9½, while the Pitcairn Islands are -8½.. Some higher latitude countries use daylight saving time for part of the year, typically by changing clocks by an hour. Many land time zones are skewed toward the west of the corresponding nautical time zones. This also creates a permanent daylight saving time effect.

Coordinated Universal Time (UTC)

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)

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.

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 (DST)

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.

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