Coordinated Universal Time, abbreviated UTC, is the basis for all time zones, which span from 12 hours behind to 14 hours ahead of UTC. TimeTemperature.com provides a map of all the time zones.
Before the advent of time zones, local officials set the time based on the position of the sun in the sky, resulting in slightly different time zones in each city and town. Nineteenth-century scholars divided the world into 24 time zones along latitudinal lines, so that each zone experienced daylight during roughly the same hours. Some countries practice daylight savings time to account for seasonal changes in daylight hours, resulting in more than 24 distinct time zones. Countries agreed to the convention of time zones to simplify business and travel around the world.
After the world adopted time zones, scientists still calculated UTC based on the position of the sun, but, as of 2015, they use more technical monitoring and calculation systems to define UTC. Scientists maintain UTC by not observing daylight savings time and using leap seconds to account for small variations in the Earth's rotation speed. Scientists found that the Earth's rotation is gradually slowing, so in order to maintain an accurate UTC, they must continually monitor and account for slight changes.