The Difference Between UTC and GMT
UTC and GMT are both references for world time — so how do they differ? We break down the origins of each standard and how they're used in practice.
What is UTC?
UTC (Coordinated Universal Time) is the official reference for time worldwide. It is based on atomic clocks, and every time zone is expressed as an offset ahead of or behind UTC.
How is it different from GMT?
GMT (Greenwich Mean Time) is an older time standard based on the prime meridian that passes through the Royal Observatory in Greenwich, England. In everyday use, UTC and GMT are treated as almost the same, but strictly speaking UTC is a scientific standard based on atomic clocks, while GMT is a historical standard based on astronomical observation.
- UTC: atomic-clock based, international standard
- GMT: astronomy based, the name of standard time in the UK and some other regions
Reading time with offsets
For example, Seoul is UTC+9 and New York is UTC−5 (or −4 when daylight saving time is in effect). The time difference between two cities is calculated from the difference in their offsets.
In practice, writing everything relative to UTC first and then adding or subtracting each region's offset leads to fewer mistakes.