Twice a year, the United States and Europe both change their clocks — but not on the same day. The gap between those two transitions creates a two-to-three week window during which the offset between any US timezone and any European timezone is one hour different from what it normally is. For teams that coordinate across the Atlantic, this window is a reliable source of missed calls and incorrect scheduling.
In 2026, the United States moves its clocks forward on Sunday 8 March. Europe moves its clocks forward on Sunday 29 March. For the three weeks between those two dates, New York is four hours behind London instead of the usual five. A standing weekly call scheduled for "4pm London, 11am New York" becomes "4pm London, 12pm New York" — but only for those three weeks, and only if everyone remembers that the shift has happened.
The same gap occurs in autumn. The US moves its clocks back in early November. Europe moves back in late October. For one week in between, the offset flips again.
The particularly damaging characteristic of this window is its invisibility. A calendar invite created before the transition and accepted by attendees in both regions will display the correct local time for each person — but based on the offset at the time the invite was created, not the offset that will apply on the day of the event. Whether the meeting time survives the transition intact depends entirely on how the calendar software handles timezone offsets, and behaviour varies between platforms.
The only reliable defence is to store and communicate meeting times in UTC or in a fully-specified offset (e.g. UTC−5, not EST) rather than in a timezone abbreviation that shifts. When a time is expressed as a named timezone, its meaning changes when DST changes. When it is expressed as a UTC offset, it does not.
During the March and November windows, it is worth double-checking every standing meeting with international attendees. The cost of that check is five minutes. The cost of missing it can be significantly higher.