Study 3 of 5
How Daylight Saving Breaks Your Calendar
More than a third of the world's city pairs — 37.3% — see their shared working hours change between January and July, for no reason other than daylight saving. A recurring meeting that works in winter can quietly drift by an hour or two by summer, and neither party moved.
Last updated July 2026
ClockJumper studies are produced with AI assistance and human review. · research methodology
- 37.3% of all city pairs (9,223 of 24,753) have a different workday overlap in July than in January.
- 122 of the 223 cities never change their clocks — across Asia, the Gulf, most of Africa, and elsewhere. Only 101 observe daylight saving.
- Every one of the six most-disrupted corridors in the dataset involves Santiago, each swinging a full two hours between seasons — the signature of hemispheric opposition.
- Most of the disruption is a clean one-hour move — 35.0% of all pairs shift by exactly 60 minutes — but a small punishing tail shifts further.
Two hours of overlap, gained or lost
The six Santiago corridors with the largest January-vs-July swing. Clocks in the north and south move in opposite directions — the workday quietly rearranges twice a year.
Of the 24,753 city pairs, 9,223 shift their overlap between the two seasons. The cause is uneven adoption: 101 of the 223 cities observe daylight saving and change their clocks twice a year, while 122 — across Asia, the Gulf, most of Africa, and elsewhere — never do. When one side of a pair springs forward and the other holds still, the window between them moves. Fifteen of the 42 timezone regimes shift; the other 27 are anchored. Most of the disruption is a clean one-hour move — 35.0% of all pairs shift by exactly 60 minutes — but a small, punishing tail shifts further.
The Santiago signature
That tail has a signature, and it points at one city: Santiago. Every one of the six most-disrupted corridors in the dataset involves the Chilean capital, each swinging a full two hours between seasons. The reason is hemispheric. Southern-hemisphere daylight saving runs on the opposite calendar to the north — when Europe and North America spring forward in March, Chile is falling back. So a Santiago–Europe or Santiago–US pair doesn't just absorb one side's clock change; it absorbs both ends moving in opposite directions, often within the same few weeks. Atlanta and Santiago, for instance, swing from six hours of overlap in January to eight in July; Athens and Santiago drop from three hours to one.
The takeaway: a calendar of danger windows
The practical takeaway is a calendar of danger windows. The gaps around March/April and October/November — when northern and southern clocks change weeks apart — are when standing meetings silently break. Teams that span the equator, or that pair a DST-observing country with one that doesn't, should expect their overlap to move twice a year and re-check it each spring and autumn. The 122 cities that never change their clocks are the stable anchors; the cross-hemisphere pairs are the ones to watch.
Full methodology is on the methodology page. The fix isn't to memorise transition dates. It's to anchor recurring meetings to a single zone, confirm the local time on both ends around the changeover weeks, and lean on tools that compute the real offset for the actual date — which is what the Clockjumper converter does. Download the dataset to see the January-vs-July figure for any pair.
Figures: January vs. July 2026, 24,753 pairs.
- How many cities still observe daylight saving?
- In this set of 223, 101 observe DST. The other 122 — across Asia, the Gulf, most of Africa, and elsewhere — never change their clocks.
- Why does my overlap with another city change in summer?
- If either city observes daylight saving (and they shift on different dates, or only one of them shifts), the gap between your clocks changes, and so does the part of the workday you share.
- Which cities are easiest to schedule with year-round?
- The ones that never change their clocks — across Asia, the Gulf and much of Africa — because the offset to them is constant in every season.
Explore the live data
The numbers on this page are a static snapshot. Use the live tools below to check the current offset, overlap, and daylight-saving status for any city or pair.