UTC vs GMT: What the Difference Is and Which One Your Clock Uses

Coordinated Universal Time (UTC) is based on International Atomic Time (TAI), while Greenwich Mean Time (GMT) is the Sun’s average time at Greenwich, and London’s winter offset from UTC is zero until Sunday, March 28, 2027. The first part is from the National Institute of Standards and Technology (NIST), whose leap-second page was updated Oct. 28, 2026; the second is from Royal Museums Greenwich. Below: the difference in plain terms, a table of London’s offset for every period through 2027, and what each term means for your devices and UK law. A one-page printable is at the end.

What is the difference between UTC and GMT?

They come from different places. NIST says: “Coordinated Universal Time (UTC) is based on International Atomic Time (TAI).” Royal Museums Greenwich says “The Royal Observatory Greenwich is the home of Greenwich Mean Time (GMT).” It describes GMT as “the yearly average (or ‘mean’) of the time each day when the Sun crosses the Prime Meridian,” and notes that “in 1884 the Greenwich Meridian was recommended as the Prime Meridian of the World.”

	UTC	GMT
Based on	International Atomic Time (NIST)	The yearly average of when the Sun crosses the Prime Meridian (Royal Museums Greenwich)
Leap seconds	“Occasionally, 1 s is added to the UTC time scale” (NIST)	Not mentioned on the Royal Museums Greenwich page
In UK law	Not mentioned in the Interpretation Act 1978, section 9	The default time in Acts of Parliament (Interpretation Act 1978, section 9)

What is a leap second, and will they end?

NIST: “Occasionally, 1 s is added to the UTC time scale. This second is called a leap second.” The reason, in NIST’s words: “Leap seconds are used to keep the difference between UT1 and UTC to within ±0.9 s.” NIST’s Internet Time Service says its servers handle a leap second “by stopping the clock for one second at 23:59:59.”

The International Bureau of Weights and Measures (BIPM) published Resolution 4 of the 27th General Conference on Weights and Measures (CGPM) in 2022. It “decides that the maximum value for the difference (UT1-UTC) will be increased in, or before, 2035.” That is a change to the allowed gap, not a ban on leap seconds, and this story does not report any later outcome.

What time is GMT now in the UK?

Since Oct. 25, 2026, UK clocks show GMT, the same reading as UTC. GOV.UK lists the 2026 changes as “Clocks go forward on 29 March and back on 25 October,” and 2027 as “Clocks go forward on 28 March and back on 31 October.” Under the Summer Time Act 1972, section 1, summer time is “one hour in advance of Greenwich mean time,” running from “one o’clock, Greenwich mean time, in the morning of the last Sunday in March” to “one o’clock, Greenwich mean time, in the morning of the last Sunday in October.” On the clock on the wall, that is a move forward at 1 a.m. and back at 2 a.m. (the same instant as 1 a.m. GMT).

So the UK is not on GMT all year. From March 28 to Oct. 31, 2027, London runs on British Summer Time (BST), which is UTC+1.

London’s offset from UTC through 2027, and the gap to Eastern Time

This table is our computation from the UK dates above and the U.S. change dates. U.S. clocks change on the second Sunday in March and the first Sunday in November (15 U.S.C. 260a), which gives March 14 and Nov. 7, 2027. Our story on the week the UK and U.S. changes did not line up in October 2026 explains the first row.

Period (Eastern Time dates)	London time	London offset	London ahead of Eastern Time (ET)
Oct. 25 to Nov. 1, 2026	GMT	UTC+0	4 hours
Nov. 1, 2026 to March 14, 2027	GMT	UTC+0	5 hours
March 14 to March 28, 2027	GMT	UTC+0	4 hours
March 28 to Oct. 31, 2027	BST	UTC+1	5 hours
Oct. 31 to Nov. 7, 2027	GMT	UTC+0	4 hours
Nov. 7 to Dec. 31, 2027	GMT	UTC+0	5 hours

The two short four-hour windows in 2027 are the weeks when only one side of the Atlantic has changed its clocks. Dates are Sundays: March 14, March 28, Oct. 31 and Nov. 7, 2027 all fall on a Sunday.

Which one does your phone use?

NIST’s Internet Time Service says: “The time is always sent as Coordinated Universal Time (UTC).” A clock that sets itself from a service like that starts from UTC, and your zone’s offset is added on top: UTC-5 for Eastern Time in winter, UTC+0 for London in winter. NIST names only its own service here; the pages we cite do not say how a given phone, airline ticket or weather forecast shows its time, so check the zone label printed beside the time.

For UK legal documents the rule is specific. Section 9 of the Interpretation Act 1978 says that, subject to section 3 of the Summer Time Act 1972, “whenever an expression of time occurs in an Act, the time referred to shall, unless it is otherwise specifically stated, be held to be Greenwich mean time.”

Printable: London UTC offsets, 2026 to 2027

The one-page sheet lists the six periods above with the Eastern Time gap, adds a short checklist for meetings set across the March 14 to March 28, 2027 window, and prints the GOV.UK and NIST wording. Pair it with the 2027 printable yearly calendar and our page on when daylight saving time starts and ends. The March printable calendar shows both 2027 change weeks. For how to write a time without the 12 a.m. mix-up, see is 12 noon a.m. or p.m., and for date formats what International Organization for Standardization (ISO) 8601 is.