Study 3 of 4
How Daylight Saving Quietly Breaks Your Calendar
Twice a year, the overlap between thousands of cities shifts — sometimes by two hours, sometimes in the wrong direction. We measured exactly how much, and found that most of the world isn't on the same schedule it was last season.
Last updated June 2026
- More than a third of all city pairs (36.6%) have a different workday overlap in July than in January — 5,076 of 13,861 pairs shift.
- Yet most cities don't change their clocks at all: 90 of our 167 cities (54%) stay on the same offset year-round. Only 77 observe daylight saving.
- The biggest swings come from the southern hemisphere, where DST runs in the opposite season. Santiago's overlap with the US East Coast jumps from six hours to a full eight in the northern summer.
- Daylight saving doesn't always help. San Francisco ↔ Tokyo loses its single shared hour in July, and Los Angeles ↔ Sydney drops from three hours to one.
The meeting map that quietly redraws itself twice a year
Same corridors, January and July. Dots joined by a line show how the workday overlap slides when clocks move — greens gain hours, reds lose them.
Most people experience daylight saving as one annoying lost hour of sleep. For anyone scheduling across borders, it's something stranger: the working relationship between cities silently re-arranges itself twice a year, and not everyone moves at once.
The headline is the mismatch. A clear majority of the world's major cities — 90 of our 167, or 54% — never change their clocks. Most of Asia, the Gulf, Africa and a growing list of others stay put all year. But the 77 cities that do shift are enough to disturb 36.6% of every possible pairing. More than a third of the corridors a global team runs on are not on the same overlap in July that they were in January.
When summer helps: the southern-hemisphere effect
The largest swings in the entire dataset come from cities whose summer is everyone else's winter. Santiago, Chile springs forward as the northern hemisphere falls back, and the gap to North America compresses dramatically: Santiago ↔ New York runs six hours of overlap in January but a full eight in July — from a workable block to complete alignment. The same opposite-season swing shows up across Chile's pairings with the US and Canada, every one of them gaining roughly two hours in the northern summer.
When summer hurts: the corridors that get worse
Daylight saving is not a scheduling gift. Because the northern and southern hemispheres move in opposite directions, some long-haul lanes lose ground exactly when you'd expect them to gain it. Los Angeles ↔ Sydney collapses from three shared hours to one between January and July, as California springs forward while Sydney falls back — the two cities pulling apart instead of together. San Francisco ↔ Tokyo loses its only shared hour entirely, dropping from one to zero, because Japan doesn't observe DST and California's shift moves the wrong way for it.
Even the average tells the story quietly: across all pairs, mean overlap nudges from 2.96 hours in January to 3.11 in July — a small global gain that hides thousands of individual corridors moving in both directions at once.
The hidden cost
The expensive part isn't the shift itself — it's the three weeks a year when it's only half-done. The US, EU, and Australia all change clocks on different dates, so for stretches of March, April, October and November the offset between two cities is briefly something it's neither normally nor seasonally. A recurring 4pm call set in spring can land an hour early or late for a fortnight before snapping back, and nobody updated the invite because nobody's clock visibly changed. The cities that stay fixed all year — Tokyo, Dubai, most of India and China — are, perversely, the easy ones to schedule with precisely because they never move.
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.
FAQ
- How many cities still observe daylight saving?
- In this set of 167, only 77 do. The other 90 — 54% — keep the same UTC offset all year.
- 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.