For three weeks a year the US and Europe are out of step

If you work across the Atlantic, there is a stretch of March where things stop lining up. Calls start at odd moments. A deadline someone computed by hand lands an hour off. Two systems disagree about when something ran.

It is not a bug in any of them. For about three weeks, the gap really is four hours rather than five.

The exact window

In 2026: the US changes on 2026-03-08, the UK and EU on 2026-03-29.

Between those dates, New York is on summer time and London is not.

There is a second, shorter window in autumn running the other way: Europe falls back on 2026-10-25 and the US on 2026-11-01, about a week apart.

What it hits

Recurring meetings written as a fixed local time on both sides — see the weekly call that moves.

Deadlines computed in advance. Anything agreed in February for a date in the gap, converted by hand at the usual five hours, is an hour wrong.

Automated jobs and reports that assume a constant relationship. These are the worst, because they fail silently and are usually discovered weeks later when someone reconciles two sets of numbers.

On-call handovers defined in local time on both sides. Depending on the direction, you either get an hour of doubled coverage or an hour with nobody holding the pager. The second one is genuinely dangerous.

Flight and travel bookings across the window, where departure and arrival sit in different offsets from each other.

What to do, in order of value

Before the window opens, do one pass over anything recurring that crosses the Atlantic. That is the whole intervention, and it takes fifteen minutes:

1. Recreate recurring invites with a zone attached, not a fixed offset or a time typed into the description.
2. Move any automated schedule to UTC. UTC has no transitions, so a job defined in it does not care about any of this.
3. For on-call rotations specifically, check that your shifts still cover twenty-four hours in both configurations. A rotation designed against a five-hour gap can have an hour-shaped hole when the gap is four.

During the window, if something looks an hour out, this is the first thing to check rather than the last. It is almost always the answer, and knowing that saves an afternoon of debugging.

Why it is three weeks and not one

It used to be one. The US extended its daylight saving period in 2007, moving the spring change from early April to the second Sunday in March. Europe did not follow, and the gap widened from a week to three.

The EU voted in 2019 to abolish the twice-yearly change altogether, but member states could not agree on which time to settle on. The vote stalled and the existing rule remains in force, so this is not going away in the near term.

Related: Every March our weekly call moves · The three weeks the US and Europe are out of step

Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.