Introduction:
Since the Earth has a spherical shape, the position of the Sun in the sky depends on the observer's location. Therefore, each place on Earth has a different solar time, and indeed used a different time until the middle of the 19
th century. Because of the confusion it created, during that period many countries started adopting a single time over the whole country or a large area of it. They defined local time as an offset of an integral number of hours with respect to Greenwich Mean Time (GMT), the time of London. The local time was usually set to be the closest integral offset of GMT to the local solar time. This created time zones all over the world.
Coordinated Universal Time (UTC) is GMT measured by atomic clocks, and one day is defined as 24 hours. However, since the rotation of the Earth varies slightly, some days may last for a little more than 24 hours, and after those differences accumulate, one second is added to UTC so it gets closer to GMT. This happens no more than once a year, usually after the last second of a year. Because of this slight difference between UTC and GMT, most countries have changed their definition of local time to be an offset of UTC, instead of GMT. For practical purposes this difference is negligible and countries can be considered to be in the same time zone even if their times are not based on the same definition.
By the way, the abbreviation UTC does not come from any language, it is rather a mixture of the abbreviations of the term in English (
Coordinated Universal Time, CUT) and French (
Temps Universel Coordonné, TUC).
Many people learn that time zones are strips of 15° between meridians, ajusted to match the borders of countries, states, provinces or other administrative divisions. This is an oversimplified view of time zones, since many countries are located mostly or even totally outside their ideal time zones, such as Argentina and China. Also, a few countries have an offset to UTC of halves or quarters of hours. The 15° time zones only occur in the oceans.
The International Date Line (IDL) is an imaginary line where not the time, but the day changes. It corresponds roughly to meridian 180°, located in the Pacific Ocean. It is ajusted to keep all of Alaska, Russia and New Zealand on the same day. The IDL used to split Kiribati into two days, so in 1995 it decided to observe the same day in the entire country, therefore changing the IDL. The IDL and time zones are not fixed by any international organization or treaty, they only reflect the time that is locally observed, therefore Kiribati effectively changed the IDL by changing its time zones.
Daylight saving time (DST), also called summer time, occurs when a country chooses to add usually one hour to its local time, during the months when sunset is later than the average time, around summer. This is done in order to use more sunlight and save electricity. Countries or parts of countries near the equator do not use daylight saving time, since in these regions the time of sunset does not vary much throughout the year.
A list of time zones is never accurate without at least approximate daylight saving information. For example, Sydney is usually said to be in UTC+10, so someone unaware of DST in Australia would assume Sydney is always 10 hours ahead of London. However, this is true only for a few hours of the whole year, as Australia and Europe usually start and end DST on the same dates, but when one starts it the other ends it and vice-versa. Hence, around summer in the Southern Hemisphere, Sydney is 11 hours ahead of London, and around summer in the Northern Hemisphere, the difference is 9 hours. A similar situation occurs between the regions of North and South America that observe DST. For example, New York is in UTC-5, and São Paulo is in UTC-3. The United States observe DST from March to November, while Brazil usually observes it from October to February. So the offset between those two cities may be of 1 hour (most of the year, when only the United States are in DST), 2 hours (a few weeks, when both or none are in DST) or 3 hours (a few months, when only Brazil is in DST).
