UTC and Time Zone Offsets: How to Calculate Time Differences
Every time zone on Earth is defined by how far it is from a single reference: Coordinated Universal Time, or UTC. Once you understand offsets, you can work out the time difference between any two places in a few seconds.
What is UTC?
UTC is the world's primary time standard. It is kept by atomic clocks and does not change with the seasons, which makes it a stable reference for aviation, computing, finance and science. 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.
Calculate a time difference in three steps
- Find both offsets for the date you care about. Offsets can change with daylight saving time, so always check the specific date.
- Subtract one offset from the other. The result is the number of hours (and minutes) between the two places.
- 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.
Try it: use Time Compare to enter a date and see the exact local time and offset for any city, including daylight saving changes.
Related guides
Daylight Saving Time Explained: Why Time Differences Change
Which countries use daylight saving time, when clocks change in the US, Europe and Australia, and why gaps between cities shift for a few weeks each year.
How to Schedule Meetings Across Time Zones
A practical checklist for planning calls across time zones: overlap windows, fair rotation, clear invitations and common mistakes.
Unusual Time Zones: Half-Hour Offsets, the Date Line and More
Why some countries use 30- or 45-minute offsets, how UTC+14 and UTC−12 can put three dates on Earth at once, and why time zones follow borders instead of maps.