Time zone

The apparent position of the Sun in the sky, and thus solar time, varies by location due to the spherical shape of the Earth.

In the 19th century, as transportation and telecommunications improved, it became increasingly inconvenient for each location to observe its own solar time.

By 1855, 98% of Great Britain's public clocks were using GMT, but it was not made the island's legal time until August 2, 1880.

In time, the American government, influenced in part by Abbe's 1879 paper, adopted the time-zone system.

[14] It was a version proposed by William F. Allen, the editor of the Traveler's Official Railway Guide.

For example, the border between its Eastern and Central time zones ran through Detroit, Buffalo, Pittsburgh, Atlanta, and Charleston.

Italian mathematician Quirico Filopanti introduced the idea of a worldwide system of time zones in his book Miranda!, published in 1858.

He proposed 24 hourly time zones, which he called "longitudinal days", the first centred on the meridian of Rome.

The proposal divided the world into twenty-four time zones labeled A-Y (skipping J), each one covering 15 degrees of longitude.

[21] By about 1900, almost all inhabited places on Earth had adopted a standard time zone, but only some of them used an hourly offset from GMT.

[22] All nations currently use standard time zones for secular purposes, but not all of them apply the concept as originally conceived.

The consequences, in some areas, can affect the lives of local citizens, and in extreme cases contribute to larger political issues, such as in the western reaches of China.

[29] Conversion between time zones obeys the relationship in which each side of the equation is equivalent to UTC.

Ships may decide to adjust their clocks at a convenient time, usually at night, not exactly when they cross a certain longitude.

In the mid-1970s the Netherlands, as other European states, began observing daylight saving (summer) time.

One reason to draw time zone boundaries far to the west of their ideal meridians is to allow the more efficient use of afternoon sunlight.

As a result, in summer, solar noon in the Spanish city of Vigo occurs at 14:41 clock time.

This westernmost area of continental Spain never experiences sunset before 18:00 clock time, even in winter, despite lying 42 degrees north of the equator.

In Midwestern states, like Indiana and Michigan, those living in Indianapolis and Detroit wanted to be on the same time zone as New York to simplify communications and transactions.

[39] A more extreme example is Nome, Alaska, which is at 165°24′W longitude – just west of center of the idealized Samoa Time Zone (165°W).

Nevertheless, Nome observes Alaska Time (135°W) with DST so it is slightly more than two hours ahead of the sun in winter and over three in summer.

Countries around the equator usually do not observe daylight saving time, since the seasonal difference in sunlight there is minimal.

[48][49] They allow a program to fetch the system time as UTC, represented as a year, month, day, hour, minute, second, and millisecond;[50][51] Windows 95 and later, and Windows NT 3.5 and later, also allow the system time to be fetched as a count of 100 ns units since 1601-01-01 00:00:00 UTC.

This does not provide a solution for more complex daylight saving variations, such as divergent DST directions between northern and southern hemispheres.

ECMA-402, the standard on Internationalization API for JavaScript, provides ways of formatting Time Zones.

The standard module datetime included with Python stores and operates on the time zone information class tzinfo.

The third party pytz module provides access to the full IANA time zone database.

VisualWorks provides a TimeZone class that supports up to two annually recurring offset transitions, which are assumed to apply to all years (same behavior as Windows time zones).

[65][66] Timekeeping on Mars can be more complex, since the planet has a solar day of approximately 24 hours and 40 minutes, known as a sol.

Earth controllers for some Mars missions have synchronized their sleep/wake cycles with the Martian day, when specifically solar-powered rover activity occurs.