The Weeks the CME Clock Slips in the UK

You sit down in London at two in the afternoon, the index futures wake up, and your day starts. Then one Monday in March you sit down at two and the open already happened. An hour ago. Nothing on the platform broke, nobody changed the contract, and the volume profile you were building is an hour further along than it should be. The United States moved its clocks and the United Kingdom did not, and for a few weeks every year the American session lands an hour earlier on your kitchen wall than it did the previous Friday.

It catches people out every single year. It catches out people who have been trading MNQ from Manchester for a decade. The fix is trivial once you understand it, and the understanding is worth twenty minutes because the same misalignment quietly corrupts session templates, time filters, backtests and news alarms all at once.

Why the clock slips at all

The UK runs on Greenwich Mean Time in winter, which is UTC+0, and British Summer Time in the warm half of the year, which is UTC+1. Chicago runs on Central Standard Time, UTC-6, and Central Daylight Time, UTC-5. Put those side by side and the gap between London and Chicago is six hours, in winter and in summer alike. Eight thirty in the morning in Chicago is half past two in the afternoon in London. That is the number most UK traders have burned into their head, and for roughly forty seven weeks of the year it is correct.

The problem is that the two countries do not flip on the same weekend. GOV.UK states the British rule plainly: the clocks go forward one hour at 1am on the last Sunday in March and back one hour at 2am on the last Sunday in October. That rule lives in the Summer Time Order 2002, which is the instrument that gave the UK its current dates.

The American rule is different and, since 2005, wider. The National Institute of Standards and Technology sets it out: US daylight saving time begins at 2:00 a.m. on the second Sunday of March and ends at 2:00 a.m. on the first Sunday of November, under the Energy Policy Act of 2005. The Americans spring forward earlier and fall back later than you do.

So there are two stretches every year when one side has already moved and the other has not. During those stretches the gap between London and Chicago is five hours instead of six, and everything on the CME calendar arrives an hour early by your clock.

The two windows, and why one is longer

The spring window opens on the second Sunday of March, when the United States moves, and closes on the last Sunday of March, when you move. Its length depends on where the Sundays fall. In 2026 the US goes forward on 8 March and the UK on 29 March, which is a three week window. In 2027 the US goes forward on 14 March and the UK on 28 March, which is only two weeks. Some years it is two, some years three. You cannot memorise a number, you have to look at the calendar.

The autumn window is shorter and more consistent. The UK falls back on the last Sunday of October, the US on the first Sunday of November, and that is almost always a single week. In 2026 the UK goes back on 25 October and the US on 1 November, so the misalignment runs Monday 26 October to Friday 30 October. In 2027 it is 31 October to 7 November, so the odd week is 1 to 5 November.

The autumn week is the nastier one, oddly, because it is short. Three weeks in March is long enough that you adapt and then have to un adapt. One week in late October or early November is short enough that plenty of people never notice, and simply spend five sessions turning up late to their own open.

The calendar in local time

Here is the whole of 2026 for a UK trader, using the equity index regular trading hours open at 8:30 a.m. Chicago time as the reference point. That is the same instant as the 9:30 a.m. New York cash open.

Stretch of 2026	London	Chicago	Gap	Index RTH open, London time
1 January to 7 March	GMT	CST	6 hours	14:30
8 March to 28 March	GMT	CDT	5 hours	13:30
29 March to 24 October	BST	CDT	6 hours	14:30
25 October to 31 October	GMT	CDT	5 hours	13:30
1 November to 31 December	GMT	CST	6 hours	14:30

And 2027, so you can put both in the diary now.

Stretch of 2027	London	Chicago	Gap	Index RTH open, London time
1 January to 13 March	GMT	CST	6 hours	14:30
14 March to 27 March	GMT	CDT	5 hours	13:30
28 March to 30 October	BST	CDT	6 hours	14:30
31 October to 6 November	GMT	CDT	5 hours	13:30
7 November to 31 December	GMT	CST	6 hours	14:30

Read it once and the pattern is obvious. There is a normal state, six hours, and there are two short deviations, five hours. Nothing else happens all year.

Every time on your day that moves

The open is the one everybody notices, but it is not the only thing that shifts. Everything anchored to a US clock moves together, and if you trade around data releases the early ones matter more than the open does. Here is the full set, in Chicago time, with both London translations.

Chicago time	What it is	London, normal 6 hour gap	London, 5 hour gap weeks
17:00 Sunday	Globex reopens for the week	23:00	22:00
07:30	The main US macro slot: CPI, payrolls, jobless claims, retail sales	13:30	12:30
08:30	Index futures regular hours open, US cash open	14:30	13:30
09:00	Second data wave: ISM, sentiment surveys, JOLTS	15:00	14:00
09:30 Wednesday	EIA crude oil inventories	15:30	14:30
13:00	FOMC statement on decision days	19:00	18:00
15:00	Index futures regular hours close, US cash close	21:00	20:00
16:00	Daily Globex halt before the next session	22:00	21:00

Crude and gold have their own rhythm on top of this. The energy pit reference open is 8:00 a.m. Chicago time and the crude settlement window runs into the early Chicago afternoon, so a CL trader in the UK loses an hour on both ends during the gap weeks too. Metals traders who anchor to the old COMEX gold open at 7:20 a.m. Chicago time go from 13:20 London to 12:20. Confirm the exact session windows for the contract you actually trade on the CME Group trading hours page, which also carries the holiday schedule, because holiday closes and early closes are a separate trap and they are all published in Central Time.

Set the platform to Chicago and stop thinking about it

The correct answer to all of this is not to be cleverer about conversions. It is to stop converting. Set your charting platform's time zone to Central Time, US and Canada, and leave it there permanently.

The logic is simple. Every session boundary that matters to a futures trader is defined by the exchange in Central Time. The daily halt, the weekly reopen, the regular hours window, the settlement, the holiday early closes. If your charts are already in that time zone, none of those boundaries ever move. Your 8:30 vertical line is on the open in January and in the middle of March and in the last week of October. The clock changes become somebody else's problem.

What actually changes twice a year is only this: what time you personally need to be at the desk, measured by the clock on your phone. That is a single fact to update, twice a year, rather than a whole chart environment to re audit.

If you insist on keeping charts in UK time, then at least fix the session template so it is defined in exchange time, and accept that you will be doing arithmetic in your head for four weeks a year.

What quietly breaks if you do not

The reason this is worth a weekend of attention is that a one hour slip does not announce itself. It degrades things.

• Session templates. If a template is defined in local UK time, the regular hours box drawn on your chart is an hour off during the gap weeks. Your opening range is wrong, your prior day high and low may be computed from the wrong block, and your value area is measured over a window that includes an hour of pre market and excludes the last hour of the cash session.
• Time filters in strategies and indicators. Anything that says "only take signals between 09:00 and 11:00" is defined in whatever time zone the platform is in. During the gap weeks a locally defined filter starts an hour late relative to the market, which in practice means you skip the first hour, the one with the volume.
• Backtests. This is the sneaky one. A backtest run across a March or November boundary with locally defined filters silently mixes two different windows. The result is not wrong enough to look wrong, which is worse than obviously broken.
• Alarms and calendars. Phone alarms follow your local clock, so they keep ringing at 14:20 while the open has moved to 13:30. Economic calendars that auto convert to your browser time zone are usually fine, but ones where you manually typed the times are not.

The Sunday open and the edge of the week

There is a second order effect on the week boundary that is worth a sentence. Globex reopens on Sunday at 5:00 p.m. Chicago time, which is 23:00 on Sunday night in London most of the year and 22:00 during the gap weeks. That is late either way, and most UK traders are not watching it, but it defines where a trading day starts for volume profile and for daily bar construction.

The consequence is that a "daily" bar on a futures chart does not correspond to a UK calendar day. It starts the previous evening, London time. If you are comparing a futures daily bar to a cash index daily bar, or to anything published on a UK date basis, those are different objects. That is true all year, but the gap weeks are when people usually notice it, because that is when the mismatch changes size and something that was silently consistent becomes visibly inconsistent.

The changeover weekends themselves also have a wrinkle: the Sunday of a US clock change is a Sunday on which the session reopens, and the hour skipped or repeated happens in the middle of the night in Chicago, well after the reopen. Data vendors handle it, but if you keep your own logs or spreadsheets with local timestamps, expect one row that looks strange.

Will this ever stop

Probably not soon, and you should plan on it continuing.

The European Union spent years on a proposal to end seasonal clock changes, and it has been sitting unresolved since 2019 with no agreed date. The UK is no longer part of that process anyway, and any change here would need its own legislation amending the arrangements in the Summer Time Order. In the United States there is a recurring push for permanent daylight time that has repeatedly passed one chamber and gone nowhere. Neither side has actually moved.

The useful thing to notice is what would happen if one of them did. If the UK went to permanent BST and the US kept its current rule, the gap would be five hours for most of the year and six for a stretch, exactly the same problem with the sign reversed. If the US went permanent and the UK kept changing, you would get two gap windows again, in different weeks. The only arrangement that removes the problem entirely is both sides freezing, which nobody is proposing in a coordinated way.

Which is the real argument for the Chicago clock. It is not a workaround for this year's calendar. It is the setup that survives whatever either legislature does next, because it ties your charts to the only clock the exchange actually cares about.

What to do before the next changeover

Short list, twenty minutes on a Saturday.

• Switch the platform time zone to Central Time and confirm the regular hours session box lands where you expect on a historical chart.
• Open every custom session template and every strategy or indicator with a time filter, and check whether the times in them are exchange time or local time. Rewrite the local ones.
• Put the four dates in your calendar: the two US changeovers and the two UK ones, for this year and next. They are in the tables above.
• Move your phone alarm, or better, set it relative to the open rather than to a fixed hour.
• On the first Monday of a gap window, be at the desk an hour early rather than an hour late. If you are wrong you lose an hour. The other error costs you the open.

None of this makes you a better trader. It just stops a calendar quirk from charging you four weeks of bad opens every year, which is a cheap thing to fix and an embarrassing thing to keep paying for.

This article is general information about market hours and platform configuration, not financial advice or a recommendation to trade any instrument. Trading leveraged futures carries a high risk of loss, including losses larger than your initial deposit, and exchange hours and holiday schedules can change, so confirm current session times with CME Group and with your broker before relying on them.

us market open time change uk

Educational content, not investment advice. Trading futures involves substantial risk of loss and is not suitable for every investor. Any reference to Algomyth results is hypothetical, simulated over historical data, and shown in full on the performance page; past performance does not guarantee future results.