The 1884 Meridian Conference
Greenwich Mean Time exists because of a political decision made in 1884. Twenty-five nations sent delegates to Washington, D.C. for the International Meridian Conference, and after weeks of debate, they voted to place the Prime Meridian at the Royal Observatory in Greenwich, London. France abstained — they wanted Paris — and didn't officially adopt the Greenwich meridian until 1911. Santo Domingo (now the Dominican Republic) was the only nation to vote against. The final tally was 22 in favor, 1 against, 2 abstentions.
The choice of Greenwich wasn't random. Britain's Royal Navy had already been using Greenwich as the reference for nautical charts, and about 70% of the world's shipping already relied on maps based on the Greenwich meridian. Practicality won. It didn't hurt that the British Empire was the dominant naval and economic power of the era — the meridian going through London was as much a reflection of geopolitics as it was of navigation standards.
There were competing proposals. The French delegation pushed for a "neutral" meridian (conveniently defined relative to Paris). Others suggested the Azores or the Bering Strait. But the sheer inertia of existing charts and navigational practice made Greenwich the pragmatic choice.
GMT vs UTC: They're Not the Same Thing
People use GMT and UTC interchangeably. Technically, they shouldn't. Here's why:
| Aspect | GMT | UTC |
|---|---|---|
| Basis | Earth's rotation (astronomical) | Atomic clocks (SI seconds) |
| Precision | Can drift as Earth's rotation slows | Accurate to nanoseconds |
| Leap Seconds | Not defined by leap seconds | Uses leap seconds to stay within 0.9s of solar time |
| Modern Usage | Colloquial / legal (UK law) | Scientific / technical standard |
| Offset | UTC+0 (by definition) | The reference (±0) |
In practice, the difference between GMT and UTC is under a second. For scheduling meetings, it doesn't matter. For satellite navigation or scientific instrumentation, it absolutely does. If you're writing software, use UTC.
The confusion runs deep even in professional contexts. I've reviewed API documentation that labels timestamps as "GMT" when they mean UTC, or worse, when they actually mean UK local time (which includes BST in summer). If a financial API returns a timestamp tagged "GMT" in July, is it really UTC+0, or is it actually BST (UTC+1)? You'd be surprised how often the answer is "the developer didn't think about it."
The Leap Second Question
UTC uses leap seconds to keep atomic time aligned with the Earth's gradually slowing rotation. Since 1972, 27 leap seconds have been inserted — always on June 30 or December 31, at 23:59:60 UTC. That extra second has caused real headaches in computing: Reddit went down during the 2012 leap second, and several airlines reported check-in system failures in 2015.
In November 2022, the General Conference on Weights and Measures (CGPM) voted to abolish leap seconds by 2035. The plan is to let UTC diverge from solar time gradually, potentially introducing a "leap minute" in a century or so. This hasn't happened yet, but it means the GMT-vs-UTC distinction may become even more academic in the coming decades. For software developers, the takeaway is: use UTC, and make sure your date/time libraries handle leap seconds if you're doing anything precision-sensitive.
GMT, being defined by astronomical observation rather than atomic clocks, never had the leap second concept baked in. It just followed the Earth's rotation, wobbles and all. That's part of why the scientific community moved to UTC in 1960 — they needed a timescale that was both uniform (atomic) and roughly aligned with solar time (leap seconds).
Countries on GMT+0 (Year-Round)
Several countries observe UTC+0 without any daylight saving adjustment:
- Iceland — uses GMT year-round, no DST. Reykjavik's IANA code is
Atlantic/Reykjavik. Despite being far enough north for significant daylight variation (4-5 hours of light in December vs 21+ hours in June), Iceland chose not to adopt DST. - West Africa — Ghana, Ivory Coast, Senegal, Mali, Mauritania, Guinea, Sierra Leone, Gambia, Guinea-Bissau, Liberia, Burkina Faso, Togo, and São Tomé and Príncipe all observe UTC+0 with no DST
- Portugal (Madeira and Azores exception) — mainland Portugal and Madeira are on WET/WEST (UTC+0/UTC+1 with DST), while the Azores are UTC−1/UTC+0
The UK itself is on GMT only in winter. From late March to late October, it switches to British Summer Time (BST), which is UTC+1. Ireland does the same, calling its summer offset Irish Standard Time (IST) — yes, the same abbreviation as India Standard Time, another delightful source of confusion.
The BST Relationship
The United Kingdom's relationship with GMT is seasonal. The Interpretation Act 1978 defines GMT as the civil time for the UK during winter. In summer, the Summer Time Act advances clocks by one hour to BST. So London is GMT from late October to late March, and BST (GMT+1) the rest of the year.
This means "London time" and "GMT" aren't synonymous for half the year. I see this mistake constantly in international scheduling tools — someone sets a meeting for "10 AM GMT" in July, and participants in London show up at 11 AM local time because they're on BST. The fix is simple: say "London time" and let the software resolve it, or use the IANA identifier Europe/London.
For reference, the 2026 UK clock changes: clocks spring forward on Sunday, March 29 (1:00 AM to 2:00 AM), and fall back on Sunday, October 25 (2:00 AM to 1:00 AM). These follow the EU pattern — last Sunday of March and last Sunday of October — even though the UK is no longer in the EU.
GMT in Software and APIs
In JavaScript, Date.prototype.toUTCString() historically returned a string containing "GMT" (like "Tue, 18 Mar 2026 12:00:00 GMT") even though it's actually UTC. The ECMAScript spec originally required "GMT" in the output for backward compatibility. This is a perfect example of how the two terms get conflated at the language level.
In Python, datetime.timezone.utc is the way to go. The pytz.timezone('GMT') approach works but is less explicit. For HTTP headers like Date, Expires, and Last-Modified, the RFC 7231 spec requires "GMT" in the date format — not "UTC." So even though UTC is the technically correct term, the web's foundational protocols still say "GMT." Standards are full of these fossils.
If you're building an API, here's my strong recommendation: always return timestamps in ISO 8601 format with a Z suffix (e.g., 2026-03-18T14:30:00Z). The "Z" stands for "Zulu time," the military/aviation term for UTC+0. It's unambiguous, parseable by every major language, and sidesteps the entire GMT-vs-UTC naming debate.
Why "Greenwich"?
The Royal Observatory in Greenwich was founded in 1675 by Charles II to improve navigation at sea. The first Astronomer Royal, John Flamsteed, was tasked with cataloging star positions to help sailors determine longitude — the great unsolved problem of the age. For over two centuries, the observatory was Britain's primary center for astronomical observation and timekeeping.
The observatory's transit instrument defined the meridian line — longitude 0°. Thousands of ships set their chronometers to Greenwich time before sailing. The time ball on the observatory roof, installed in 1833, dropped at exactly 1:00 PM every day so ships in the Thames could synchronize their clocks. You can still see the time ball today — it still drops daily, though now it's a tourist attraction rather than a navigational tool.
By the time the 1884 conference happened, Greenwich was already the de facto global reference. The observatory's telegraphic time signals, broadcast from the 1850s onward, were the world's first "time service" — a Victorian-era NTP server, if you will.
Today the observatory is a museum. The actual work of maintaining UTC is done by the Bureau International des Poids et Mesures (BIPM) in Paris, which averages readings from over 400 atomic clocks worldwide. The irony of the French connection isn't lost on anyone.
The Prime Meridian Has Moved (Sort Of)
Here's a fun fact that surprises most visitors to the Royal Observatory: if you stand on the brass line in the courtyard that marks the "Prime Meridian" and check your GPS, you'll find you're at approximately 0°0'5.3" West — about 102 meters from where GPS says 0° longitude is. The discrepancy exists because the original meridian was defined by a transit instrument observing stars, which is affected by local gravity variations. Modern geodetic systems (WGS 84, used by GPS) define 0° longitude slightly differently. The historical line and the GPS line don't quite agree.
This doesn't affect time zones at all — timezone boundaries are political, not geodetic. But it's a nice reminder that even something as fundamental as "where is zero longitude" depends on which measurement system you're using.
Frequently Asked Questions
Is GMT the same as UTC?
For everyday purposes, yes — they share the same offset (UTC+0). Technically, GMT is based on Earth's rotation while UTC is based on atomic clocks. The difference is sub-second. In software and scientific contexts, UTC is the correct term.
Does the UK always use GMT?
No. The UK uses GMT from late October to late March, then switches to British Summer Time (BST, UTC+1) for the rest of the year. "London time" changes depending on the season.
What countries are on GMT+0 year-round?
Iceland, Ghana, Ivory Coast, Senegal, Mali, Mauritania, Guinea, Sierra Leone, Gambia, Guinea-Bissau, Liberia, Burkina Faso, and Togo all observe UTC+0 without DST.
Why is it called "Greenwich" Mean Time?
It's named after the Royal Observatory in Greenwich, London, where the Prime Meridian (0° longitude) was established. The observatory was the historical center of British navigation and timekeeping, and its meridian was adopted internationally in 1884.
What is the IANA code for GMT?
There is no single IANA code for "GMT." Use the city-based identifier for the location you need, such as Europe/London for the UK or Atlantic/Reykjavik for Iceland. The generic Etc/GMT exists but doesn't handle DST transitions.
Is GMT+0 the same as UTC+0?
In practice, yes — both represent the same offset with zero hours difference from the reference meridian. The technical distinction is that UTC is defined by atomic clocks while GMT is based on astronomical observation, but the difference is sub-second and irrelevant for scheduling.
Does Iceland use GMT?
Yes. Iceland observes UTC+0 (equivalent to GMT) year-round with no daylight saving time. The IANA identifier is Atlantic/Reykjavik. This makes Iceland one of the most predictable time zones for international scheduling.
Sources
- International Meridian Conference Proceedings, 1884
- Royal Observatory Greenwich — History of the Prime Meridian
- Bureau International des Poids et Mesures (BIPM) — UTC Definition
- UK Interpretation Act 1978 — Definition of GMT