What Is a UTC Offset?
A UTC offset is the amount of time by which a given time zone differs from UTC. It is written as UTC+X or UTC−X, where X is the number of hours (and optionally minutes) ahead of or behind UTC.
- UTC+9: Seoul and Tokyo — 9 hours ahead of UTC
- UTC−5: New York in winter — 5 hours behind UTC
- UTC+0: London in winter, Reykjavik, Accra — no offset
- UTC+5:30: Mumbai — 5 hours and 30 minutes ahead
How to Convert Between Time Zones
To convert from one time zone to another, first convert to UTC, then add the destination offset:
- Start with local time: 3:00 PM Seoul (UTC+9)
- Convert to UTC: 3:00 PM − 9 hours = 6:00 AM UTC
- Convert to New York (UTC−5): 6:00 AM − 5 hours = 1:00 AM New York
Alternatively, subtract Seoul's offset from New York's offset: −5 − (+9) = −14 hours. New York is 14 hours behind Seoul.
Positive vs. Negative Offsets
Positive offsets (UTC+) are east of the prime meridian. Negative offsets (UTC−) are west. As you travel eastward from Greenwich, England, the offset increases by one hour roughly every 15 degrees of longitude.
UTC Offset vs. Time Zone
These terms are related but not identical. A time zone has a name and identity (e.g., "Eastern Time"); a UTC offset is a numeric value. One time zone can have two different offsets throughout the year if it observes daylight saving time. New York is UTC−5 in winter (EST) and UTC−4 in summer (EDT).
ISO 8601 and UTC Offsets
The international date-time standard ISO 8601 uses UTC offsets directly in timestamps:
2025-06-15T09:00:00+09:00— 9:00 AM in a UTC+9 zone2025-06-15T00:00:00Z— midnight UTC (Z= UTC+0)2025-06-15T04:00:00-05:00— 4:00 AM in UTC−5
This format eliminates ambiguity and is the recommended way to exchange date-time data across systems.