Guides: how clocks, calendars and the sky actually work
The reference pages on this site tell you what time it is somewhere. These guides explain why: why zones are offset by half an hour, why the day and night line curves, why noon on the clock is not noon by the sun. Each one is written and checked by hand, dated, and links to the tool that lets you see the thing being described.
12 guides, most recently reviewed October 5, 2026
Time zones and clocks
Why the world's clocks disagree, by exactly how much, and how to work across them.
- How time zones work, and why there are more than 24 of themWhy Earth is cut into hour-wide bands, who decided where the lines go, why China runs on one clock and France on twelve, and where the offsets come from.
- UTC vs GMT: what the difference is, and when it mattersGMT is a time zone defined by the sun over Greenwich. UTC is an atomic standard kept within a second of it. Here is where the two part ways and which one to write.
- Daylight saving time explained: who changes their clocks, when, and whyWhich countries move their clocks, the dates the United States, Europe and Australia use, why the southern hemisphere runs opposite, and how to check any place.
- Why some time zones are offset by 30 or 45 minutesIndia, Iran, Newfoundland and central Australia keep half-hour offsets; Nepal and the Chatham Islands keep 45-minute ones. The geography behind each odd clock.
- Why CST, IST and BST mean different things in different placesCST is Central, China or Cuba Standard Time. IST is India, Ireland or Israel. PST is Pacific or Philippine. How to read an ambiguous code and what to write instead.
Calendars and dates
Weeks, leap years, the date line, and the arithmetic behind the date calculators.
- The International Date Line: how one planet keeps three dates at onceWhy crossing the Pacific can skip or repeat a day, why the line zigzags around Kiribati and Samoa, and the two hours each day when Earth is on three calendar dates.
- How to schedule a meeting across time zones without getting it wrongA method that survives daylight saving, half-hour offsets and the date line: anchor to UTC, name cities not codes, check the date, and find the overlap.
- ISO week numbers explained: why January 1 can be in week 52How ISO 8601 numbers the weeks of a year, why some years have 53, why the first days of January can belong to the previous year, and how the US convention differs.
- Leap years explained: why 2000 was one and 2100 will not beThe year is 365.2422 days long, so calendars add a day roughly every four years. The full Gregorian rule, why 1900 and 2100 are skipped, and how we got here.
Sun and sky
What sunrise, twilight, solar noon and the moving day and night line really mean.
- Civil, nautical and astronomical twilight, and the golden hour, explainedSunset is not the end of daylight. The three twilight bands are defined by how far the sun is below the horizon, and their length depends on latitude and season.
- Why the day and night line curves, and how fast it movesThe terminator is a circle on a sphere that looks like a wave on a flat map. How Earth's tilt bends it through the year, and how fast it moves.
- Why noon on the clock is not solar noon: the equation of timeClock noon and the sun's highest point can differ by over an hour. The three causes: your place inside the zone, daylight saving, and the equation of time.
How these guides are kept honest
Every guide carries the date a person last re-read it, and that date is set by hand, never by the build. Rules that governments can change, such as daylight saving dates, are described as the rules in force on the review date, and the live pages they link to are computed from the time zone database in your browser rather than from the guide's text, so the clocks stay right even if the prose lags. If you find an error, the corrections page explains what to send. The equations behind the sun and moon numbers are listed on the methodology page.