How Time Zones and UTC Work
Coordinated Universal Time (UTC) is the primary time standard by which the world regulates clocks. It replaced Greenwich Mean Time (GMT) as the global standard in 1972. UTC is determined by atomic clocks and is independent of Earth's rotation; occasional leap seconds are added (or theoretically subtracted) to keep UTC within 0.9 seconds of astronomical time (UT1). All time zones are expressed as UTC offsets: UTC+5:30 (India), UTC−5 (US Eastern Standard), UTC+9 (Japan). Some time zones use non-integer offsets: India (UTC+5:30), Nepal (UTC+5:45), and Iran (UTC+3:30).
Daylight Saving Time (DST) shifts clocks forward 1 hour in spring and back in autumn in many countries, changing the UTC offset by +1 hour. The US, EU, Canada, and Australia use DST; Japan, India, China, and most of Africa do not. The IANA Time Zone Database (tz database) is the definitive reference for all world time zones and DST rules, updated several times per year as countries change their rules.
