Time zones are regions of the globe that observe a uniform standard time for legal, commercial, and social purposes. Time zones tend to follow the boundaries of countries and their subdivisions because it is convenient for areas in close commercial or other communication to keep the same time.
Many locations observe daylight saving time (DST), also known as summer time, which involves advancing clocks forward by one hour during summer months and returning back to standard time during winter months. The dates of DST changes can vary between countries.
UTC is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude and is not adjusted for daylight saving time. It is effectively a successor to Greenwich Mean Time (GMT).