Eastern European Summer Time in the context of "Time in the Republic of Ireland"

Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.

Because of Earth's uneven angular velocity in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the Sun crosses the Greenwich Meridian and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy described by the equation of time. Noon GMT is the annual average (the arithmetic mean) moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".

Kaliningrad Time (KALT; Russian: калининградское время, romanized: kaliningradskoye vremya) is the time zone two hours ahead of UTC (UTC+02:00) and one hour behind Moscow Time (MSK−1). It is used in Kaliningrad Oblast.

Until 2011, Kaliningrad Time was identical to Eastern European Time (UTC+02:00; UTC+03:00 with daylight saving time). On 27 March 2011, Russia moved to permanent DST, switching Kaliningrad time permanently to UTC+03:00. On 26 October 2014, this law was reversed but daylight saving time was not reintroduced, so Kaliningrad is now permanently set to UTC+02:00.

Central European Summer Time (CEST, UTC+02:00), sometimes referred to as Central European Daylight Time (CEDT), is the standard clock time observed during the period of summer daylight-saving in those European countries which observe Central European Time (CET; UTC+01:00) during the other part of the year. It corresponds to UTC+02:00, which makes it the same as Eastern European Time, Central Africa Time, South African Standard Time, Egypt Standard Time and Kaliningrad Time in Russia.

Eastern European Time (EET) is one of the names of the UTC+02:00 time zone, 2 hours ahead of Coordinated Universal Time. The zone uses daylight saving time, so that it uses UTC+03:00 during the summer.

A number of African countries use UTC+02:00 all year long, where it is called Central Africa Time (CAT), although Egypt and Libya also use the term Eastern European Time.

Europe spans seven primary time zones (from UTC−01:00 to UTC+05:00), excluding summer time offsets (five of them can be seen on the map, with one further-western zone containing the Azores, and one further-eastern zone spanning the Ural regions of Russia and European part of Kazakhstan). Most European countries use summer time and harmonise their summer time adjustments; see Summer time in Europe for details.

The time zones actually in use in Europe differ significantly from uniform zoning based purely on longitude, as used for example under the nautical time system. The world could in theory be divided into 24 time zones, each spanning 15 degrees of longitude. However, due to geographical and cultural factors, it is not practical to divide the world so evenly, and actual time zones may differ significantly from those based purely on longitude. In Europe, the widespread use of Central European Time (CET) causes major variations in some areas from solar time. Based on solar time, CET would range from 7.5 to 22.5°E. However, for example Spain (almost entirely in the Western hemisphere) and France (almost entirely west of 7.5°E, as illustrated in the map below) should theoretically use UTC, as they did before the Second World War. The general result is a solar noon which is much later than clock noon, and later sunrises and sunsets than should theoretically happen. The Benelux countries should also theoretically use GMT.

Western European Time (WET, UTC+00:00) is a time zone covering parts of western Europe and consists of countries using UTC+00:00 (also known as Greenwich Mean Time, abbreviated GMT). It is one of the three standard time zones in the European Union along with Central European Time and Eastern European Time.

The following Western European countries and regions use UTC+00:00 in winter months:

Western European Summer Time (WEST, UTC+01:00) is a summer daylight saving time scheme, 1 hour ahead of Greenwich Mean Time and Coordinated Universal Time. It is used in:

• the Canary Islands
• Portugal (including Madeira but not the Azores, which uses UTC+00;00)
• the Faroe Islands

The following countries also use the same time zone for their daylight saving time but use a different title:

During British Summer Time (BST), civil time in the United Kingdom is advanced one hour forward of Greenwich Mean Time (GMT), in effect changing the time zone from UTC+00:00 to UTC+01:00, so that mornings have one hour less daylight, and evenings one hour more.

BST begins at 01:00 GMT every year on the last Sunday of March and ends at 02:00 BST on the last Sunday of October. The starting and finishing times of daylight saving were aligned across the European Union on 22 October 1995, and the UK retained this alignment after it left the EU; both BST and Central European Summer Time begin and end on the same Sundays at 02:00 Central European Time, 01:00 GMT. Between 1972 and 1995, the BST period was defined as "beginning at two o'clock, Greenwich mean time, in the morning of the day after the third Saturday in March or, if that day is Easter Day, the day after the second Saturday in March, and ending at two o'clock, Greenwich mean time, in the morning of the day after the fourth Saturday in October."