Convert GMT to CST Greenwich Mean Time ↔ Central Standard Time Free Online Secure
Type a time in GMT and read it in CST straight away. The offset is −6 h, the arithmetic wraps at midnight, and the answer says which day it lands on.
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Through the day — Convert GMT to CST (Greenwich Mean Time ↔ Central Standard Time)
| GMT | CST |
|---|---|
| 00:00 | 18:00 |
| 03:00 | 21:00 |
| 06:00 | 00:00 |
| 09:00 | 03:00 |
| 12:00 | 06:00 |
| 15:00 | 09:00 |
| 18:00 | 12:00 |
| 21:00 | 15:00 |
Where these units come from
Until the middle of the nineteenth century every town kept its own noon, set by the sun overhead, and nobody minded that the next town was four minutes behind. Railways ended that: a timetable cannot be written for a hundred local noons. Britain put its railways on Greenwich time in 1847, and in 1884 the International Meridian Conference in Washington chose Greenwich as the prime meridian and sketched the twenty-four-hour scheme we still use. Greenwich Mean Time was the world's reference until 1972, when Coordinated Universal Time replaced it: UTC is kept by atomic clocks and tied back to the turning Earth by leap seconds. The abbreviations on this page name standard time — the winter setting — which is why each one is a fixed number of hours rather than something that drifts through the year.
Where you meet them
A time zone becomes real the moment two people in different places have to agree on one moment: a call between offices, a flight that lands before it took off, a market that opens while you are asleep, a deadline written in somebody else's afternoon. Server logs, train and flight schedules and anything stamped with a time are kept in UTC precisely so that the question can always be settled, no matter whose clock was on the wall.
Questions people ask
What about summer time?
The offsets here are standard-time offsets, the ones a zone keeps in winter. Where summer time is observed, the same places move one hour further on for part of the year and take a different name: Central European Time becomes Central European Summer Time, Eastern Standard Time becomes Eastern Daylight Time. The switching dates are not the same in every country, and the two hemispheres change in opposite months, so for a date inside a summer-time period add an hour to whichever side observes it.
Why isn't every difference a whole number of hours?
Because offsets are decided by governments, not by geometry. India is half an hour off the hour, at UTC+05:30; Nepal is three quarters, at UTC+05:45; the Chatham Islands are at UTC+12:45. Any of these can be changed by law, and several countries have moved their offset within living memory.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
What Greenwich time looks like in the middle of North America
The Central time zone is where the continent keeps its heartland clock. Chicago, Dallas, Houston, Minneapolis, Kansas City, New Orleans, Winnipeg and Mexico City all share it in winter, and so do the capitals of several Central American countries all year round. If a British colleague proposes a call at 16:00 GMT and you work in any of these places, you need to convert GMT to CST. Subtract six hours: 16:00 GMT is 10:00 CST, 12:00 GMT is 06:00 CST, and 04:00 GMT is 22:00 CST on the previous day.
The sections below set out the rule, give two lookup tables, describe both zones, and explain why the gap narrows to five hours for four weeks of the year. They also cover Mexico’s abolition of summer time, the Canadian prairies, Midwestern football Saturdays, long-haul travel and the usual mistakes. The arithmetic is performed in your browser, so nothing you type goes anywhere.
How to convert GMT to CST
Take off six hours
Central Standard Time is UTC−6 and Greenwich Mean Time is UTC+0, so Central clocks trail Greenwich by six hours. Subtract six from the GMT hour each time you convert GMT to CST. If the answer is below zero, add twenty-four and step back one date. 03:40 GMT, for instance, gives −2:20, which is 21:40 CST the previous day.
Eight readings worth memorising
00:30 GMT is 18:30 CST the previous day. 06:15 GMT is 00:15 CST. 09:00 GMT is 03:00 CST. 12:00 GMT is 06:00 CST. 15:45 GMT is 09:45 CST. 18:00 GMT is 12:00 CST. 21:30 GMT is 15:30 CST. 23:45 GMT is 17:45 CST. Only the first row falls on the earlier date.
A half-day trick
Six hours is a quarter of a day. When you convert GMT to CST in your head, read the GMT clock as a quarter-turn ahead of Central time. Noon in Greenwich is dawn in Chicago, and midnight in Greenwich is six in the evening in Chicago, still the working day’s end.
GMT from the Central viewpoint
The time of the evening news
For someone in Texas or Illinois, GMT runs well ahead. By the time Chicago sits down to lunch at noon, London is turning towards evening at 18:00. A British morning message arrives in the small hours of the Central night, which is why early replies from the Midwest to Britain are rare.
One caution about London
Britain keeps GMT only from the last Sunday of October to the last Sunday of March. Between 29 March and 25 October 2026, London’s clocks show BST, an hour ahead, so a British colleague who says “GMT” in July may be referring to the London wall clock. When the source is a person describing a meeting, ask which clock they have in mind.
CST in detail
The places on Central time
In the United States the zone covers most of Texas, Louisiana, Oklahoma, Kansas, Nebraska, the Dakotas, Minnesota, Iowa, Missouri, Arkansas, Wisconsin and Illinois, plus parts of other states. In Canada it takes in Manitoba and, with a twist, Saskatchewan. In Mexico it covers the capital and most of the country, and in Central America it includes Guatemala, El Salvador, Honduras, Nicaragua and Costa Rica.
A winter name
Strictly speaking, CST is the winter label. From the second Sunday of March to the first Sunday of November the United States and Canada use Central Daylight Time, CDT, which is UTC−5. People often write CT, which means whichever of the two is in force.
Other CSTs
China Standard Time (UTC+8) and Cuba Standard Time (UTC−5) share the abbreviation. In this converter CST is the North American Central zone at UTC−6.
Six hours and the working day
The narrow window
A London office open from 09:00 to 17:00 GMT is open from 03:00 to 11:00 CST. A Chicago office open from 09:00 to 17:00 CST is open from 15:00 to 23:00 GMT. The two overlap between 15:00 and 17:00 GMT, which is 09:00 to 11:00 CST: a window of just two hours.
How teams use it
Distributed teams that “convert GMT to CST” calculation for a weekly call place their live meetings inside that window, most often at 15:30 or 16:00 GMT. Everything else is handled asynchronously, with the London side writing updates before it leaves and the Chicago side reading them at the start of its day.
Crossing the Atlantic
A flight from London that leaves at 12:00 GMT and lasts about eight and a half hours lands at 20:30 GMT, which is 14:30 CST. The traveller seems to arrive in the middle of the same afternoon, with six hours of the day “gained”.
Four weeks of five-hour gaps
Why they happen
North America starts summer time on the second Sunday in March and ends it on the first Sunday in November. Europe starts on the last Sunday in March and ends on the last Sunday in October. In 2026 the North American changes are on 8 March and 1 November, and the British ones on 29 March and 25 October.
The odd weeks
From 8 to 28 March, Chicago is on CDT (UTC−5) while London is on GMT, so the gap is five hours. From 25 to 31 October, London is back on GMT while Chicago is still on CDT, and the gap is again five hours. The rest of the year the gap is six hours: GMT to CST in winter and BST to CDT in summer.
What to do with the calculator
The converter above subtracts six hours. If your date falls in one of the four odd weeks, add one hour to the result. For example, 15:00 GMT on 12 March is 10:00 CDT, not 09:00 CST.
Mexico after summer time
A law passed in 2022
Mexico ended daylight saving time in October 2022, with a few border municipalities excepted. As a result, Mexico City sits on UTC−6 throughout the year. The gap to London is six hours in winter and seven in summer, when London moves to BST. The gap to Chicago is zero in winter and one hour in summer, when Chicago moves to CDT.
What business partners should do
A supplier in Monterrey who schedules a call for 09:00 local time will be at 15:00 GMT in winter but at 16:00 BST in summer. Anyone who needs to convert GMT to CST for such a call should look at the date first and then at the labels.
The Canadian prairies
Manitoba and Saskatchewan
Winnipeg, in Manitoba, follows Central time with the usual summer shift. Regina and Saskatoon, in Saskatchewan, stay on UTC−6 all year. That means that in July Regina is in step with Mountain time in Alberta rather than with Winnipeg, a quirk that surprises visitors.
A worked case
At 14:00 GMT on a January afternoon, both Winnipeg and Regina read 08:00. In July, 14:00 UTC is 09:00 CDT in Winnipeg and 08:00 CST in Regina. A calendar that lists “Central” without a city is ambiguous in summer.
College football Saturdays
Noon in the Midwest
Saturday afternoon games in the Big Ten and Big 12 often kick off at noon Central, which is 18:00 GMT in the autumn after the clocks go back. A kick-off at 11:00 a.m. CST starts at 17:00 GMT. The evening slot of 7:00 p.m. CST is 01:00 GMT on Sunday.
Watching from Britain
British fans of American football typically follow the early Sunday window, which is the evening slot in the United States. With a printed schedule beside you, a quick convert GMT to CST lets you tell whether to watch live or wait for highlights.
Central America and the tourist calendar
Check-in and transfers
Guatemala, Honduras, Nicaragua, El Salvador and Costa Rica keep UTC−6 all year. A hotel check-in at 15:00 local time is 21:00 GMT. A transfer scheduled for 06:00 local time leaves at 12:00 GMT, which is early afternoon in Bucharest in winter.
Flights from Europe
An evening arrival at San José at 19:30 CST corresponds to 01:30 GMT the next day. Travellers booking onward connections should check which date the airline prints on the ticket.
Conferences in Chicago
Planning a trip around the sessions
A keynote that begins at 9:00 a.m. CST starts at 15:00 GMT. A closing session at 4:30 p.m. CST ends at 22:30 GMT. Delegates who fly in from Britain the day before have time to recover before the first session.
Virtual attendance
People watching online from Europe can use 15:00 to 22:30 GMT as their window. Panels held after 6:00 p.m. CST begin at midnight GMT, and recorded access is usually the only practical option.
The Midwestern farm calendar
Reports and weather
Planting and harvest depend on weather forecasts, which are released several times a day in local time. A forecast published at 5:00 a.m. CST appears at 11:00 GMT. A frost warning valid until 8:00 a.m. CST expires at 14:00 GMT.
International buyers
A buyer in Britain who tracks the market will find it easier to place orders if the key times are in a single scale. After each “convert GMT to CST” calculation, write the result next to the original to build a personal timetable for the week.
Every hour in one table
GMT to CST
00:00 GMT is 18:00 CST (previous day). 01:00 GMT is 19:00 CST (previous day). 02:00 GMT is 20:00 CST (previous day). 03:00 GMT is 21:00 CST (previous day). 04:00 GMT is 22:00 CST (previous day). 05:00 GMT is 23:00 CST (previous day). 06:00 GMT is 00:00 CST. 07:00 GMT is 01:00 CST. 08:00 GMT is 02:00 CST. 09:00 GMT is 03:00 CST. 10:00 GMT is 04:00 CST. 11:00 GMT is 05:00 CST. 12:00 GMT is 06:00 CST. 13:00 GMT is 07:00 CST. 14:00 GMT is 08:00 CST. 15:00 GMT is 09:00 CST. 16:00 GMT is 10:00 CST. 17:00 GMT is 11:00 CST. 18:00 GMT is 12:00 CST. 19:00 GMT is 13:00 CST. 20:00 GMT is 14:00 CST. 21:00 GMT is 15:00 CST. 22:00 GMT is 16:00 CST. 23:00 GMT is 17:00 CST.
Reading the table
Each entry pairs a Greenwich hour with the matching Central hour and flags when the date moves. The rows follow the same six-hour subtraction that the converter uses when you convert GMT to CST.
Examples with minutes
Ten more readings
01:05 GMT is 19:05 CST (previous day). 03:20 GMT is 21:20 CST (previous day). 05:50 GMT is 23:50 CST (previous day). 07:35 GMT is 01:35 CST. 10:10 GMT is 04:10 CST. 13:25 GMT is 07:25 CST. 16:40 GMT is 10:40 CST. 19:55 GMT is 13:55 CST. 22:15 GMT is 16:15 CST. 23:05 GMT is 17:05 CST.
Minutes are untouched
Because six hours is a whole number of hours, the minutes in the result are the same as in the question. A mismatch means a typing slip.
The other American zones from Greenwich
A ladder
Subtract five hours for Eastern, six for Central, seven for Mountain and eight for Pacific. Alaska is nine and Hawaii ten. “Central” always means six, never five.
One moment, five clocks
When London reads 18:00, New York reads 13:00, Chicago 12:00, Denver 11:00 and Los Angeles 10:00. An invitation that lists all five lets everyone skip a separate convert GMT to CST.
Dated examples through the year
January
On Friday 16 January, 16:00 GMT is 10:00 CST. Both sides are on standard time, so subtract six hours.
March
On Friday 20 March, 16:00 GMT is 11:00 CDT. The United States moved its clocks on 8 March, so subtract five hours.
August
On Friday 14 August, 16:00 BST is 10:00 CDT. Both sides are on summer time, so the gap is six hours again.
November
On Friday 6 November, 16:00 GMT is 10:00 CST. After 1 November both sides are back on standard time.
Checklist: how to convert GMT to CST
Before
Check the label
CST, CDT or CT? If it is CT and the date lies between 8 March and 1 November, it is CDT.
Check the country
CST can be Chinese or Cuban. Here it means North American Central.
After
Subtract six hours
If the GMT hour is below 06:00, add twenty-four first.
Check the date
GMT hours from 00:00 to 05:59 belong to the previous date in Chicago.
Airline hubs and connections
Chicago and Dallas as gateways
O’Hare and Dallas–Fort Worth are two of the busiest hubs in the world, and many passengers from Europe change aircraft there. An evening arrival from London at 17:30 CST left Heathrow at about 15:00 GMT. A connection to the west coast that departs at 19:15 CST leaves at 01:15 GMT on the next date, which is worth knowing if you track the flight from London.
Reading a boarding pass
Every time on a boarding pass is local. If you want to keep one running timeline, convert each leg to GMT. A “convert GMT to CST” calculation for the first departure and another for the last arrival gives you the true length of the journey.
Hospitals, clinics and remote consultations
Specialists in the Midwest
Large clinics in Minnesota, Illinois and Texas offer video consultations for patients abroad. A slot at 9:00 a.m. CST corresponds to 15:00 GMT, a convenient time for patients in Britain. A slot at 2:00 p.m. CST is 20:00 GMT, which fits patients who work during the day.
Prescriptions and reports
Reports are usually dated in local time. A discharge letter stamped 11:45 p.m. CST on a Monday is dated 05:45 GMT on Tuesday. When a foreign insurer asks for the “date of treatment”, it is useful to give both.
Festivals and trade shows in Texas
The spring calendar
Big events in Austin and Houston take place in March, when Texas is already on summer time. A panel at 10:00 a.m. CDT begins at 15:00 GMT if Britain is still on winter time, and at 16:00 BST after 29 March. The shift in the second half of the month is the source of countless missed webcasts.
Registration and badges
Badge pick-up generally opens at 8:00 a.m. local time, which is 14:00 GMT. A delegate who flew in overnight from Britain will feel as if it were already the middle of the afternoon.
Mistakes when you convert GMT to CST
Adding instead of subtracting
Chicago is behind London, so the Central number is smaller. If it is larger, you added.
Using five hours
Five hours belongs to the Eastern zone. Central needs six.
Losing the previous date
A deadline of 02:00 GMT on Friday is 20:00 CST on Thursday.
Assuming Mexico shifts too
Mexico City stays on UTC−6 all year, so in summer it is an hour behind Chicago.
Frequently asked questions
What is the difference between GMT and CST?
Six hours. CST is behind GMT.
How do I convert GMT to CST?
Subtract six hours, adding twenty-four and stepping back a date if the result is negative.
What is 15:00 GMT in CST?
09:00 CST.
Why is the gap sometimes five hours?
Because North America and Europe change their clocks on different dates, which creates four odd weeks each year.
Is my input uploaded?
No. It stays in your browser.
More time zone converters
Related pages: Convert GMT to PST, Convert GMT to WET, Convert IST to CST, Convert WET to CST, Convert CST to UTC, Convert UTC to GMT, Convert IST to PST, Convert PST to EST, Convert WET to UTC. The reverse direction is Convert CST to GMT.
In short, to convert GMT to CST subtract six hours, check the date and remember the four weeks when the gap is five. The calculation runs on your own device, with no sign-up and no upload.