Time zone
A time zone is a region on Earth that has a uniform standard time
Standard time
Standard time is the result of synchronizing clocks in different geographical locations within a time zone to the same time rather than using the local meridian as in local mean time or solar time. Historically, this helped in the process of weather forecasting and train travel. The concept...

 for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates (for example, the sun being at its highest point every day around noon), different places on the Earth need to have different clock times. Time zones have been used in modern times so similarly situated cities can keep exactly the same time, for simplicity and ease of communication.

Standard time zones could be defined by geometrically subdividing the Earth's spheroid
A spheroid, or ellipsoid of revolution is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters....

 into 24 lunes
Lune (mathematics)
In geometry, a lune is either of two figures, both shaped roughly like a crescent Moon. The word "lune" derives from luna, the Latin word for Moon.-Plane geometry:...

 (wedge-shaped sections), bordered by meridians
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...

 each 15° of longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....