A time zone keeps clocks synchronized within a region. There are 24 primary time zones, each roughly 15° of longitude wide.
Major Time Zones
- UTC+0: GMT (London)
- UTC-5: EST (New York)
- UTC-8: PST (Los Angeles)
- UTC+5:30: IST (India — offset zone)
- UTC+8: CST (Beijing, Singapore)
- UTC+9: JST (Tokyo)
DST
Daylight Saving Time shifts clocks forward 1 hour in spring ("spring forward") and back in fall. Not all regions observe DST.
Why Time Zone Boundaries Aren't Straight Lines
Although time zones are theoretically 15° of longitude wide, actual boundaries zigzag to follow country and state borders, keeping regions with close economic and social ties on the same clock — China, spanning a longitude range that would normally cover 5 time zones, uses only one nationwide for political and administrative simplicity.
UTC vs. GMT
UTC (Coordinated Universal Time) is the modern technical standard, while GMT (Greenwich Mean Time) is the historical, astronomically-defined predecessor. For nearly all everyday purposes they're interchangeable and numerically identical, but UTC is the term used in computing, aviation, and international standards.