UTC Offset Explained: UTC vs GMT, and How to Convert Time

Last updated October 14, 2026 · Written by Han with AI assistance, fact-checked against official sources.

Every time zone is defined by its distance from one reference, Coordinated Universal Time (UTC). With the two offsets in hand, the time difference between any two places is a subtraction.

What is UTC, and is it the same as GMT?

UTC is the world's primary time standard. It is kept by atomic clocks and does not change with the seasons, so aviation, computing and finance use it as a fixed reference. People often say "GMT" when they mean UTC. The two show the same clock time, but Greenwich Mean Time is also the name of a local time zone: it is what the United Kingdom uses in winter, while UTC is the technical standard itself.

How offsets are written

A time zone is described by its offset from UTC. Tokyo and Seoul are at UTC+9, which means their clocks are nine hours ahead of UTC. New York is UTC−5 in winter and UTC−4 in summer, which means its clocks are five or four hours behind UTC. A plus sign means ahead of UTC (east of Greenwich, roughly) and a minus sign means behind it.

Time zone vs UTC offset

The two words are often used as if they meant the same thing, but they do not. A UTC offset is one number that says how far a clock is from UTC at a given moment, such as UTC−5. A time zone is a place and its rules, written in software with an IANA name such as America/New_York or Asia/Kolkata. The rules say which offset applies on which date.

Some zones have one offset all year: Asia/Kolkata is always UTC+5:30. Others have two: America/New_York is UTC−5 in winter and UTC−4 in summer. So "UTC−5" tells you the time in New York in January but not in July. If you save a future meeting, store the time zone name, not the offset, so the time still comes out right after a clock change.

Calculate a time difference in three steps

1. Find both offsets for the date you care about. Offsets can change with daylight saving time, so always check the specific date.
2. Subtract one offset from the other. The result is the number of hours (and minutes) between the two places.
3. Apply it to the time and check the date. The place that is further east is ahead, and it may already be on the next calendar day.

Worked example

Suppose it is 15:00 in Tokyo (UTC+9) and you want the time in New York in winter (UTC−5). First convert to UTC: 15:00 − 9 hours = 06:00 UTC. Then apply New York's offset: 06:00 − 5 hours = 01:00. So it is 01:00 in New York on the same calendar day. In summer, when New York is UTC−4, the answer is 02:00.

Offsets with minutes

Not every offset is a whole number of hours. India uses UTC+5:30, Nepal uses UTC+5:45, and parts of Australia use UTC+9:30. When one of these is involved, include the minutes in your subtraction. For example, India is 3 hours 30 minutes behind Tokyo, not 3 hours or 4 hours.

Avoid ambiguous abbreviations

Abbreviations such as CST and IST look precise but are not. CST can mean China Standard Time, US Central Standard Time or Cuba Standard Time. IST can mean India, Ireland or Israel. When you share a time, write the city or the UTC offset (for example, "09:00 Chicago time" or "15:00 UTC") to remove any doubt. Software uses unambiguous IANA names such as America/Chicago or Asia/Kolkata for the same reason.

Time Compare does this subtraction for any city and date, with daylight saving changes included.