The clock change: what actually happens at 2am
One hour vanishes in spring and one hour occurs twice in autumn. Both are real, both are in the time zone database, and both break software that assumes time only moves forward.
The short answer
In spring the clock jumps from 02:00 straight to 03:00 — that hour does not exist. In autumn it reaches 02:00 and returns to 01:00 — that hour happens twice.
2am is chosen deliberately: late enough that most people are asleep, early enough that it precedes the first trains, and far enough from midnight that the calendar date never shifts.
Why 2am and not midnight
If the change happened at midnight, going back an hour would move the clock into the previous calendar date. Anything keyed to a date — contracts, licences, birth certificates, tickets — would briefly disagree about what day it was.
2am avoids that, and it also sits after most late-night services have run and before the earliest scheduled transport. In the EU the switch is defined at 01:00 UTC instead, so the whole union changes at one instant rather than in a rolling wave.
Spring: the hour that does not exist
At 02:00 local, the clock becomes 03:00. Times between them are invalid for that date — 02:30 simply never occurs.
What breaks:
- A job scheduled for 02:30 local does not run. Not late — it never fires, because the trigger time did not happen.
- Timestamps in that range are unparseable. Software must either reject them or pick a policy, and different libraries pick different ones.
- The night is an hour shorter. A night shift crossing it works seven hours, not eight.
Autumn: the hour that happens twice
At 02:00 local, the clock returns to 01:00. Every time between 01:00 and 02:00 occurs twice, an hour apart.
What breaks:
- Timestamps become ambiguous. "01:30" that night is two distinct instants. Logs from the window cannot be ordered by local time alone.
- Jobs can fire twice. Anything defined in local time that sends, charges or moves something will do it twice unless it is idempotent.
- Shifts run long. A night shift crossing it works nine hours.
Why this is a software problem, not a clock problem
Both failures come from the same wrong assumption: that local time increases monotonically and every value occurs exactly once. Neither is true twice a year.
Two rules avoid nearly all of it:
Store and schedule in UTC. UTC has no transitions. Convert to local time only for display, at the last possible moment.
Never do arithmetic on local times. "Add 24 hours" to a local timestamp is wrong on transition days — the correct answer is 23 or 25 hours of elapsed time. Do the arithmetic on the instant, then convert.
What ordinary people notice
Almost none of the above. What people notice is that the evening is suddenly dark, that they are tired for a few days after the spring change, and that one of their recurring calls with another country has moved and nobody knows why.
That last one is the three-week gap between the US and European switch dates.
Related: When does daylight saving time end? · The night with two 1:30ams
Timezone: World Clock Widget
A world clock, converter and live day/night map for iPhone — 325 cities, offline, no account.
Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.