Clocks moved forward for part of the year — on dates that do not agree between countries.
Daylight saving time moves a jurisdiction's clocks forward, usually by an hour, for part of the year. Roughly a third of the world observes it, the southern hemisphere does it in the opposite months, and the tropics largely do not bother.
The operational problem is not the change itself but that jurisdictions change on different dates. For two weeks each March the United States has sprung forward and Europe has not, so the gap between New York and London is four hours instead of five — and every recurring meeting scheduled in local time silently moves for one of the parties.
The rules also change. Legislatures adjust dates, adopt and abandon the practice, and occasionally do it at short notice, which is why time zone data is software that needs updating rather than a table that can be copied once.
The dates do not agree, and the gap between them is where recurring meetings break. All four are 2027 transitions computed from the IANA database.
| 🇺🇸 | Sunday 14 March 2027United States — New York | Clocks forwardUTC−5 becomes UTC−4. |
| 🇬🇧 | Sunday 28 March 2027United Kingdom | Clocks forwardTwo weeks later. For those fourteen days London and New York are four hours apart, not five. |
| 🇳🇿 | Sunday 4 April 2027New Zealand | Clocks backThe southern hemisphere runs the other way — ending summer time as the north begins it. |
Standard time is the offset a place keeps when daylight saving is not in force. The two alternate.
Daylight saving is the rule; the offset is what it produces. A place has one set of rules and two offsets.
See also standard time · utc offset · time zone · iana time zone · overlap window
We hold this as not stored — resolved from the IANA zone at the date in question. See what we hold.