Time zone guides and explainers

Long-form answers to the questions the calculators can't: why time zones exist, why they don't line up with the 15° lines you'd expect, and what actually happens to your body, and your calendar, when you cross a lot of them at once.

Written by Nicolai Lykke. How these guides are researched, checked and corrected is on the editorial standards page.

• The history of time zones

  How the railways and the 1884 Prime Meridian Conference gave the world standard time.

• UTC vs GMT: what's actually different

  Why both names survive, what genuinely separates them, and which to use when it matters.

• The politics of daylight saving time

  Why some countries spring forward every year while most of Africa and Asia never bother.

• The International Date Line

  How a zig-zagging line in the Pacific lets you fly west and land "a day ahead".

• Jet lag: circadian misalignment and how to beat it

  The science of why crossing time zones wrecks your body clock, and what actually helps.

• The world's strangest time zone offsets

  Nepal's +5:45, the Chatham Islands' +12:45 and Kiribati's leap across the date line.

• Why China runs on a single time zone

  One country, five geographic zones, one clock, and what that means at sunrise in Kashgar.

• The countries that span the most time zones

  France, the United States, Russia and the UK, and why overseas territories settle the contest.

• How air travel handles time zones and the date line

  Why your boarding pass shows local times, and what really happens on a date-line crossing.

• How to schedule meetings across time zones

  Finding a fair working-hours overlap, when to switch to async, and why a recurring slot drifts twice a year.

• What is UTC, and why the internet runs on it

  The world's reference clock that every time zone is measured from, what it is, how it differs from GMT, and why servers store it.

• How to find the best time to call another country

  The best time is the overlap of both sides' waking hours, how to find it, and the weekend and daylight-saving traps.

• Where does each day begin?

  Which places see each new day, and each new year, first, which see it last, and the Pacific line that decides.

• The 24-hour clock explained

  Midnight is 00:00 and there's no am/pm to get wrong, how it works, how to convert it, and why aviation and medicine insist on it.

• How remote teams work across time zones

  Designing around scarce overlap instead of fighting it: async by default, follow-the-sun handoffs, and norms that keep it fair.

• The largest time difference on Earth

  The two furthest-apart places on the clock are 26 hours apart, more than a full day. How that's possible.

• Unix time: the seconds since 1970

  Why computers keep time as one plain number, where you meet it, and the year 2038 problem waiting at the end.

• Will the clocks stop changing?

  The US and EU have both moved to end the twice-yearly change, and both stalled on the same question: which time to keep.

• How your phone always knows the right time

  The two hidden systems behind an always-correct clock: network time sync, and the time-zone database governments keep changing.

• How to convert a time between two zones without getting DST wrong

  The three questions to settle before you convert anything: which date, which direction, and whether either side is on summer time.

• Why a converted time shifts by an hour twice a year

  The gap between two cities is not a constant. Here is what moves it, when, and why the shift can last a fortnight.

• Finding a meeting time that works across three continents

  With two places you look for overlap. With three you are choosing who gets the awkward hour, so choose it deliberately.

• What counts as a reasonable hour to ask of someone

  Office hours are not the same everywhere, and neither is lunch. A practical guide to the hours worth protecting.

• Announcing an online event so nobody arrives an hour late

  Publish the moment, not your local clock reading. What to put in the announcement, and what to leave out.

• Why calendar invites sometimes shift by an hour

  An invite carries a time zone, not just a time. When that zone is wrong or missing, the meeting moves without anyone touching it.

• Choosing the cities for your world clock

  A useful world clock is not the biggest cities. It is the ones you actually wait on, arranged so you can read it at a glance.

• Reading a world clock when half the cities are on summer time

  For a few weeks each spring and autumn the familiar gaps are wrong. Here is which weeks, and which cities to distrust.

• Converting Unix timestamps in practice: logs, APIs and spreadsheets

  Seconds or milliseconds, UTC or local, and the spreadsheet formula that quietly assumes the wrong one.

• The year 2038 problem, in practical terms

  What actually breaks when a 32-bit timestamp runs out, what has already been fixed, and where the risk still sits.

• ISO week numbers explained, and why week 1 moves

  The rule is one sentence long, and it is the reason January can start in week 53 of the year before.

• Why two calendars can disagree about the week number

  ISO weeks, US weeks and broadcast weeks all count differently. Which one your colleague means depends on where they sit.

• Counting days between two dates without the off-by-one

  Inclusive or exclusive is the whole argument. Decide it first, say which you used, and the number stops being contested.

• Why a day is not always 24 hours long

  Twice a year a local day runs 23 or 25 hours. It is why date arithmetic done in hours goes wrong and date arithmetic done in days does not.

• How age is actually calculated, leap days included

  Age is a count of completed years, not a division of days. The edge cases are 29 February and the day before a birthday.

• Birthdays across time zones: when you actually turn a year older

  Move far enough east or west and your birthday starts at a different moment than it did at home. Which reading counts is a choice.

• Working days versus calendar days, and what contracts mean

  A notice period counted in working days can be half again as long as the same number of calendar days. Check which one you agreed to.

• Why a business-day count differs from country to country

  The working week is not Monday to Friday everywhere, and public holidays are local. Both change the answer.

• Adding days, weeks and months: where date arithmetic gets ambiguous

  Adding days is unambiguous. Adding months is not, and the 31st of the month is where every implementation shows its hand.

• Deadlines that land on a weekend or a public holiday

  Most rules roll the deadline forward, some roll it back, and a few do neither. Which applies decides whether you are late.

• Counting down to a moment, not to a local time

  A countdown shared across zones has to target one instant. Anchor it in UTC and every viewer sees the same number.

• Why countdown timers drift, and how to keep one honest

  A timer that counts its own ticks slowly loses the plot. One that recomputes from the target time cannot.

• Reading and writing military time without the guesswork

  Four digits, no colon, no am or pm, and a spoken form that trips up newcomers. The conversion rules in full.

• Zulu time and the NATO time zone letters

  Why UTC is called Zulu, what the other 24 letters mean, and where you still meet them outside the military.

• Timesheets: rounding rules, unpaid breaks and overtime

  Where the hours go missing is almost always rounding or breaks. Both are worth agreeing in writing before the first pay run.

• Night shifts that cross midnight, and the two odd nights a year

  A shift that ends on the next date needs care in any timesheet, and on the two clock-change nights it is genuinely 23 or 25 hours.

• Why the moon phase you see depends on your time zone

  The phase is a moment, not a day. Where you stand decides which local date that moment lands on.

• Why two calendars can give different full moon dates

  One prints the UTC date, the other your local one. Near midnight that is a whole day of disagreement about the same instant.

• Why sunrise times differ across a single time zone

  A time zone is a political band, not a line of longitude. At its western edge the sun comes up an hour later by the clock.

• Twilight explained: civil, nautical and astronomical

  Three definitions, three sun angles, three quite different amounts of usable light. Which one your app means matters.