On 8 March 2026, New York has a 23-hour day. At two in the morning the clocks jump straight to three. Half past two never happens.
On 1 November the same city has a 25-hour day. At two in the morning the clocks go back to one, so half past one happens twice, an hour apart.
A day is not 24 hours. It is however many hours that day had.
Why this breaks things
Most date code adds 86,400,000 milliseconds to get to tomorrow. On 363 days of the year that is correct. On the other two it lands an hour early or an hour late, and the bug only appears twice a year — which is exactly long enough for everyone to forget the last time.
The symptoms are familiar. A recurring 9am meeting that drifts to 8am for three weeks. A daily report that runs twice on one morning and not at all on another. An alarm that does not go off.
The two awkward questions
Ask any converter for half past two in the morning on 8 March in New York. There is no right answer, because that time does not exist. A good tool says so. A bad one silently gives you 01:30 or 03:30 and lets you find out later which.
Then ask it for half past one on 1 November. There are two right answers, an hour apart, and they are different moments. A good tool asks which one you mean.
| Date (New York) | Hours in the day | The odd hour |
|---|---|---|
| 8 March 2026 | 23 | 02:00–03:00 does not exist |
| 1 November 2026 | 25 | 01:00–02:00 happens twice |
| Any other day | 24 | — |
It is not always an hour
Lord Howe Island, off the east coast of Australia, moves its clocks by thirty minutes rather than sixty. It is the only place that does. Anything that hard-codes a one-hour jump is simply wrong there, twice a year, for ever.
Ireland is stranger still. The database models Irish time as standard in summer and running an hour behind in winter — the reverse of how Britain is described, for the same clocks. Ask software whether Ireland is on daylight saving and the honest answer depends on which way round you define the question.
This is why it is better to ask how far the clocks have been moved than whether it is summer time. A number of minutes is true in every one of these cases. A yes or no is not.
What to check in anything you rely on
- Ask for 02:30 on the spring-forward date in a place that observes it. You should be told the time does not exist.
- Ask for 01:30 on the autumn date. You should be asked which of the two you mean, or at least told there are two.
- Lay out the spring-forward day as hours. There should be 23 of them, with no hour repeated and none missing.
- Check somewhere on a half-hour offset, such as India, and somewhere on a quarter-hour one, such as Nepal.
None of this requires trusting anybody. Every browser carries the full time-zone rulebook, so the answers are sitting on the machine in front of you — the only question is whether the thing you are using bothers to ask it.