EST to GMT: Eastern Time to London

The three terms are used interchangeably, but they name different things:

• GMT (Greenwich Mean Time) began as the mean solar time at the Royal Observatory in Greenwich. Today it is the name of the UTC+0 time zone and the winter time of the United Kingdom.
• UTC is the atomic time standard from which every offset is counted, including Eastern Time's −5 and −4. Its clock reads the same as GMT; how it is kept is covered on EST to UTC.
• UK civil time is what clocks in London actually show: GMT from late October to late March and British Summer Time (BST, UTC+1) the rest of the year. British law still treats GMT as the base: under section 9 of the Interpretation Act 1978, a time in an Act of Parliament means GMT unless summer time applies.

So "EST to GMT" has two answers. If you literally mean GMT – a timestamp, or a country that keeps it all year, such as Iceland, Ghana or Senegal – the gap is 5 hours while ET is on EST and 4 hours while ET is on EDT, as the UTC offset page explains. If you mean London, the gap is 5 hours for most of the year, because both cities spring forward. The rest of this page is about London; the live Eastern Time clock shows which offset ET is on today.

By the 1880s most of the world's nautical charts already counted longitude from the Royal Observatory at Greenwich, and British railways had run on London time since the 1840s; Parliament made GMT the legal time of Great Britain in 1880. In October 1884, 41 delegates from more than two dozen countries met in Washington, D.C., for the International Meridian Conference and voted 22 to 1 – San Domingo against, France and Brazil abstaining – to adopt the meridian through Greenwich's transit instrument as the prime meridian. France did not move its legal time to Greenwich until 1911.

That decision is why Eastern Time is written "UTC−5". The zone's central meridian, 75°W, lies 75 degrees west of Greenwich, and at 15° per hour that is 5 hours. American railroads had already defined their zones in whole hours from Greenwich in 1883, a year before the conference made Greenwich the world's reference.

Britain itself once gave up GMT in winter. From 1968 to 1971 the UK stayed on GMT+1 all year in the "British Standard Time" experiment, so for three winters London was 6 hours ahead of New York. Complaints about dark mornings, loudest in Scotland, ended the trial, and the UK has followed the GMT/BST cycle since 1972.

London's office day ends at 5:00 PM, which is 12:00 PM in New York most of the year, so the shared window is 9:00 AM–12:00 PM ET (2:00–5:00 PM in London). A 10:00 AM ET call (3:00 PM London) is the usual compromise: New York has had an hour to settle in and London is not yet winding down. In the mismatch weeks London's afternoon starts an hour earlier on the Eastern clock and the window widens to 9:00 AM–1:00 PM ET (1:00–5:00 PM London).

Those weeks are also when transatlantic calls get missed. A recurring invitation created in one city's time zone keeps its local time there and moves by an hour for the other side, so a standing 10:00 AM ET call lands at 2:00 PM in London instead of 3:00 PM.

Markets overlap less than offices. The NYSE opens at 9:30 AM ET, which is 2:30 PM in London, and the London Stock Exchange closes at 4:30 PM UK time, leaving about two hours of shared trading – three during the mismatch weeks. The stock market hours page has the full schedule.