UTC (Coordinated Universal Time) is the global reference clock that
every time zone is measured against. It replaced GMT as the
international standard in 1972 and is kept precise using atomic
clocks. Airlines, military operations, scientific research, and
computer servers all rely on UTC so that events can be coordinated
across borders without ambiguity.
UTC and GMT are closely related but not identical in strict terms. UTC
is the modern time standard maintained by atomic clocks, while GMT is
technically a time zone centred on Greenwich, London. In everyday use
the two names are interchangeable — both represent offset zero — but
UTC is the correct term for the global reference. This converter
displays results as UTC/GMT to reflect that common usage.
Daylight saving time shifts local clocks forward or back by an hour in
many countries, which changes the offset between a city and UTC
throughout the year. London is UTC+0 in winter (GMT) but UTC+1 in
summer (BST); New York is UTC−5 in winter and UTC−4 in summer. Clockr
uses IANA timezone data through the browser's Intl API, so the
correct offset for your chosen date is applied automatically — no
manual adjustment needed.
Frequently asked questions
What is UTC and how is it different from GMT?
UTC (Coordinated Universal Time) is the world's primary time
standard, maintained by atomic clocks and accurate to within a
fraction of a second. GMT (Greenwich Mean Time) is a time zone
based on the sun's position at the Royal Observatory in Greenwich,
London. In practice the two are used interchangeably for offset
zero, but UTC is the technically correct term for the global
reference.
What is the current UTC offset for London?
London is UTC+0 during winter when GMT is in effect, and UTC+1
during summer when British Summer Time (BST) is active. The switch
happens on the last Sunday in March (clocks forward) and the last
Sunday in October (clocks back). Use the converter above with
London selected to see the live offset for today's date.
How do I convert UTC to my local time?
Enter the UTC time and date in the "UTC to your time" card, then
select your city from the time zone dropdown. The converter
instantly shows the equivalent local time and date, accounting
for daylight saving on that specific day. Your browser's time zone
is detected automatically as the default.
Why do clocks change between GMT and BST?
The UK observes daylight saving time to make better use of evening
daylight during the longer summer days. Clocks spring forward one
hour to BST in late March and fall back to GMT in late October.
This shifts the offset from UTC+0 to UTC+1 for roughly seven months
of the year.
Is UTC the same as Greenwich Mean Time?
For most practical purposes, yes — both represent the same offset
(zero) and are used interchangeably in aviation, broadcasting,
and computing. Strictly speaking, UTC is an atomic time standard
while GMT is a solar-based time zone, but the difference is less
than a second and irrelevant for everyday conversions.
What time zone do pilots and the military use?
Aviation and military operations worldwide use UTC (often called
"Zulu time" in aviation). Flight plans, air traffic control
clearances, and military orders are all expressed in UTC to avoid
confusion across time zones. NATO and many armed forces also use
the NATO phonetic alphabet, where Zulu represents UTC.
How is UTC time used in computing and servers?
Computer systems store and process timestamps in UTC internally,
then convert to local time only when displaying to users. Unix
timestamps, database records, log files, and API responses all use
UTC to ensure consistency across servers in different regions.
Converting UTC to local time is one of the most common tasks in
software development.