What a time zone does not tell you
A time zone tells you what a clock says. It does not tell you where the sun is, how long the day lasts, or whether a call at 9am lands in someone's morning or their night. Those are four different questions and people routinely use one number to answer all of them.
This page separates them, because almost every planning mistake we see comes from collapsing two of these into one.
The four separate things
- UTC offset. A political decision. It is set by governments, changes when they decide it changes, and can be adjusted by half or quarter hours. It tells you how to convert one clock to another and nothing else.
- Longitude. A physical fact that determines when the sun is actually overhead. Solar noon moves four minutes per degree of longitude, regardless of what the clock says.
- Latitude. A physical fact that determines how long the day is and how much it varies across the year. It has no relationship whatsoever to the time zone.
- Working hours. A cultural convention. Spain, Japan and Germany share almost nothing in when the working day starts and ends, and none of it is visible in the offset.
Offset and longitude come apart: Spain
Madrid and Lisbon are about six degrees of longitude apart — Madrid at -3.7°, Lisbon at -9.1° — which is roughly 25 minutes of real solar time. But Spain runs on Central European Time and Portugal runs on Western European Time, an hour apart. The clocks say an hour; the sun says twenty-five minutes.
The consequence shows up in solar noon. In Madrid on the December solstice the sun reaches its highest point at 13:13 local time, not 12:00. In Lisbon it is 12:35. Madrid's clock is running well ahead of its sun, which is a large part of why Spanish daily life happens later than the rest of western Europe's — the late lunch and late dinner are not purely cultural, they are partly the country living on the wrong side of a time zone boundary.
We compute this gap for every city we track. It is on the clock time versus sun time page, and the extremes are larger than most people expect.
Offset and latitude have nothing to do with each other
Tromsø and Stockholm are both on the same offset for most of the year. On the December solstice Stockholm gets 6h 05m of daylight. Tromsø gets 0h 00m — the sun does not rise at all. Same clock, entirely different day.
Run it the other way and it is just as stark. Katong & East Coast and Bangkok are an hour apart on the clock but only about 12 degrees apart in latitude, so their December daylight is within 44 minutes of each other. The clock difference is a border; the daylight difference is barely there.
Half-hour and quarter-hour zones break naive arithmetic
India, Sri Lanka, most of Iran, parts of Australia and Newfoundland run on offsets that are not whole hours, and Nepal runs on a 45-minute offset. Any calculation that rounds an offset to the nearest hour will be wrong by up to 45 minutes for these places, and the error is silent — it produces a plausible number rather than an obvious failure.
This is not hypothetical. We hit it in our own code: an overlap calculation that printed “UTC+6” for Colombo and Goa, both of which are UTC+5:30, and reported a two-hour overlap window that was actually two and a half. The full list of affected zones is on the half-hour time zones page.
Daylight saving moves the offset, not the daylight
Two cities that overlap comfortably in July can lose an hour of overlap in January because one of them observes daylight saving and the other does not, or because they switch on different dates. The southern hemisphere switches in the opposite direction from the northern, so a European–Australian pair moves by two hours across the year rather than one.
The daylight itself, of course, does not care. Moving a clock forward does not create light. It relocates a fixed amount of daylight relative to the working day, which is useful for evenings and irrelevant to the total. We work through the overlap consequences on the daylight saving overlap page.
What we do about it
Everything on this site that involves a clock uses the IANA time zone database with the rules applied for the specific date being asked about, not for today. Everything that involves the sun is computed from latitude, longitude and date using standard solar geometry. The two are kept apart deliberately, because as this page has laboured, they answer different questions. The method is on our verification page, and you can check any figure with the meeting planner.
Common follow-up questions
Does a time zone tell you how much daylight a city gets?
No — that is latitude and date. Tromsø and Stockholm sit on the same offset, and on the December solstice Stockholm gets 6h 05m of daylight while in Tromsø the sun does not rise at all.
Why is solar noon not at 12:00?
Because the clock is a political construct and the sun is not. Solar noon shifts four minutes per degree of longitude within a zone, and the equation of time adds a seasonal wobble of up to about sixteen minutes. Madrid, on Central European Time despite sitting west of the Greenwich meridian in places, runs well behind its clock.
Which countries use half-hour time zones?
India and Sri Lanka at UTC+5:30, Iran at +3:30, Afghanistan at +4:30, Myanmar at +6:30, parts of Australia at +9:30 and +8:45, Newfoundland at −3:30, and Nepal at +5:45. Any calculation that rounds offsets to whole hours is wrong for all of them.
Does daylight saving give you more daylight?
No. It moves a fixed amount of daylight relative to the working day. What it does change is time zone overlap between countries that switch on different dates or in opposite directions.