Time FAQ
The questions people ask most about time zones, daylight saving, UTC and time difference.
What is the difference between UTC and GMT?
UTC (Coordinated Universal Time) is the modern atomic time standard that the world's clocks are set against. GMT (Greenwich Mean Time) is a time zone based on the mean solar time at Greenwich. For everyday purposes they show the same time, but GMT is a time zone while UTC is a standard — and only UTC never changes.
What is a time zone?
A time zone is a region of the world that keeps the same clock time. Each zone is defined by its offset from UTC, usually in whole hours, though some regions use half-hour or 45-minute offsets. There are 24 standard zones in theory and about 38 distinct offsets in practice.
How does daylight saving time work?
Daylight saving time (DST) moves clocks forward by one hour in spring and back in autumn to make better use of evening daylight. Not every country uses it — most of Asia, Africa and South America do not — and the start and end dates differ by country and can change from year to year.
How do I work out the time difference between two cities?
Find each city's current UTC offset and subtract one from the other. For example, New York (UTC−5) and London (UTC+0) differ by five hours. The easiest way is a time zone converter, which accounts for daylight saving automatically.
Why is the time difference sometimes 30 or 45 minutes?
A few regions use offsets that are not whole hours — India is UTC+5:30, Nepal is UTC+5:45, and parts of Australia and Canada use half-hour offsets — so the difference from another city can include 30 or 45 minutes.
What is Unix time?
Unix time (also called epoch time or POSIX time) is the number of seconds that have elapsed since 1 January 1970 at 00:00:00 UTC, ignoring leap seconds. Computers use it because it is a single time-zone-independent number.
What is the ISO week number?
The ISO week number is a standard way of numbering weeks: week 1 is the week containing the first Thursday of the year, and weeks run from Monday to Sunday. It is used widely in business and scheduling across Europe.
How many time zones does the world have?
There are 24 standard time zones, one for roughly every 15 degrees of longitude. In practice around 38 different UTC offsets are in use because of half-hour and 45-minute zones, and the IANA database lists about 600 named zones including historical and alias entries.
Does the time on this site update automatically?
Yes. Clocks and countdowns update every second in your browser, and each city is rendered using its IANA time zone so daylight saving changes are applied automatically.
How accurate are the times on this site?
Times are calculated in your browser using the IANA time zone database and your device clock. They are accurate as long as your device's clock is correct; the time zone and daylight saving rules come from the standard IANA data.
Put it into practice
Now that you know the theory, try the tools: the world clock shows many cities at once, the time zone converter compares two zones, the time calculator adds or subtracts time, the Unix timestamp page shows epoch time live, and the epoch converter translates timestamps to dates. Browse time zones, countries and continents to explore further.