Zone-pair conversions

The ones people look up most, with the part most pages leave out — what changes twice a year.

PST to EST: three hours, and the one week a year it breaks

The gap between the US west and east coasts is three hours almost always — and the exception is short, predictable, and the reason a recurring call slips.

EST to PST: subtract three hours

Going east to west you lose three hours — which is why a 9am East Coast call is a 6am West Coast call, and why nobody on the west end enjoys it.

CST to EST: one hour, and why this one never moves

Central to Eastern is the most stable conversion in the United States — and the reason it is stable is exactly the reason IST to EST isn’t.

IST to EST: the conversion that moves twice a year

India is 9:30 or 10:30 ahead of New York depending on the date — and India never changes its clocks. The whole shift comes from the American side.

UTC to EST: subtract five — or four

UTC never moves, which makes it the reference every other zone is defined against. New York moves twice a year, which is where the second answer comes from.

Convert EST to GMT

Five hours in winter — but the two sides change their clocks on different weekends, so for a few weeks each year the answer is neither five nor four.

Eastern Time to GMT: what “ET” actually means

ET is not a fixed offset. It is a promise to use whichever of two offsets is correct today — which makes it the safest thing to write and the hardest to convert blind.

Time zone converter: what a good one actually does

Converting an hour is trivial. The difference between converters is entirely in the four cases that are not trivial.

Time zone calculator

Doing it by hand takes three steps and works every time — provided you handle the two things mental arithmetic reliably drops.

Time difference calculator

The difference between two places is not a fixed property of the pair — it depends on the date, and for a lot of pairs it changes twice a year.

Daylight saving time

What changes, when, who still does it, and what it reliably breaks. Seasonal — worth re-reading before March and November.

Daylight saving time: what changes, when, and what breaks

Clocks move an hour twice a year in some countries, on different dates, and the parts that break are never the clocks themselves.

When does daylight saving time start?

Different dates in different places — and the three weeks between the American and European switches are the ones that quietly break transatlantic calendars.

When does daylight saving time end?

Clocks go back, you gain an hour, and one hour of the night happens twice — which is a genuine problem for anything that timestamps things.

The clock change: what actually happens at 2am

One hour vanishes in spring and one hour occurs twice in autumn. Both are real, both are in the time zone database, and both break software that assumes time only moves forward.

Daylight saving vs standard time

Standard time is the baseline; daylight time is the summer offset. Most of the year is not standard time, which is why “EST” in July is almost always wrong.

How does daylight saving time work?

A statutory instruction to shift the civil clock, encoded in a database your phone carries, applied automatically — until two countries disagree about the date.

Which US states don’t observe daylight saving time?

Two, plus several territories — and the consequence is that those places change which coast they match, twice a year, without touching a clock.

Does Arizona observe daylight saving time?

No — which makes Phoenix the hardest US city to schedule against, because it changes which coast it matches without ever changing its clock.

Spring forward, fall back: the mnemonic and its limits

It tells you the direction and nothing else — not the date, not whether your country does it, and not which way round it is south of the equator.

Eastern daylight time, right now

UTC−4, in force from March to November, and the correct label for New York for two thirds of the year — which is exactly as long as most people spend calling it EST.

Central daylight time

UTC−5 from March to November, covering Chicago, Dallas and Houston — and sharing its abbreviation with a zone thirteen hours away.

The three weeks a year the US and Europe are out of step

The US springs forward in March. Europe waits. In between, every transatlantic recurring call is an hour off from what both calendars claim.

Which countries still change their clocks?

A shrinking minority — mostly Europe, North America and a few southern-hemisphere states. Most of the world's population has never moved a clock.

What DST does to a recurring calendar invite

Nothing visible. The entry is identical, the attendees are identical, and it happens an hour earlier for half of them — which is why nobody ever finds the cause.

Zones, abbreviations and offsets

What the letters mean, why several mean more than one thing, and what to write instead.

EST vs EDT: the difference that ruins meetings

They are not the same offset, they are never both correct, and using the wrong one is how a recurring call quietly moves by an hour.

What is EST?

Eastern Standard Time is UTC−5, it covers the winter half of the year, and for the other half the correct label is something else entirely.

What is UTC time?

The reference every other zone is defined against — atomic, unchanging, and the only sensible thing to write down when a timestamp has to survive being read elsewhere.

What does GMT mean?

Greenwich Mean Time — a time zone, not a standard, and one that stops describing British local time for seven months of every year.

UTC offsets: the full list

From UTC−12 to UTC+14 — twenty-seven hours of range across a twenty-four hour day, because several countries chose their side of the date line.

Time zone abbreviations decoded

Including the ones that mean two different things — CST is fourteen hours wide depending on which country you assume.

The tz database: the file every device quietly depends on

Why your phone knows Kathmandu is 45 minutes off, that Arizona doesn't change its clocks, and what the rules were in 1987.

How many time zones are there?

The answer is not 24 — and depending on what you count, it is 38, or about 350, or somewhere in between.

How do time zones work?

Every zone is UTC plus an offset, every offset is a political decision, and every device resolves them through the same shared database.

Why do time zones exist?

Because trains kept crashing. Every town keeping its own solar time was fine until two places needed to agree on a timetable.

The international date line, explained

It is not straight, it is not a treaty, and crossing it can put you a full day from a neighbour an hour away.

What time zone is California in?

Pacific Time — the whole state, one zone, and it does change its clocks despite a 2018 ballot measure that suggested otherwise.

China has one time zone for the whole country

Five geographic hours wide, one official clock — so in the far west the sun comes up when the clock says mid-morning, and that is the correct local time.

World clock and city time

What a world clock is for, where the built-in one stops, and how to read one at a glance.

World clock

A list of places and their local times — deceptively simple, and the differences between implementations are entirely in what they do at the edges.

World time clock

Every place on earth reading the same instant differently — and the reason the whole system works is that underneath, they are all reading the same one.

What time is it in Japan?

One zone for the whole country, no daylight saving, nine hours ahead of UTC — which makes Japan one of the easiest countries in the world to schedule against.

What time zone am I in?

Your device already knows, and it knows it as an IANA identifier rather than an abbreviation — which is the answer you actually want if anyone is going to convert it.

World clock with seconds

Seconds are the same everywhere — every civil time zone is offset by a whole number of minutes — so a ticking display is about confidence, not accuracy.

Analog world clock: reading time as a shape

A dial encodes the time as an angle, and an angle is something you recognise rather than read — which is why a wall of analog faces is scannable in a way a column of digits is not.

What time is it in another country?

The method takes ten seconds and works for anywhere — including the countries whose offsets are not whole hours and the ones that never change their clocks.

Time in different countries, compared

Laid out side by side, the pattern is not really about geography — it is about which countries move their clocks and which do not.

What time is it tomorrow, somewhere else?

For a chunk of every day, somewhere on earth is already on tomorrow — and if you are messaging them, your “today” and theirs are different dates.

A world clock widget for your iPhone home and lock screen

The best version of a world clock is one you never open — because the question it answers takes two seconds and opening an app takes longer than that.

What the iPhone’s built-in world clock can’t do

It answers the first question well and the other two not at all — an honest look at where Apple’s Clock app stops.

Maps, day and night

Where the sunlight currently is, why the line curves, and what a map shows that a list cannot.

Day and night map: where the sun is right now

One curve across a world map tells you who is awake, who is asleep, and who is about to be — and you can scrub it forward to see tomorrow.

Where is it daytime right now?

Roughly half the planet, always — but which half moves west at a thousand miles an hour, and the shape of it changes with the season.

The day/night terminator, explained

Why the line between day and night is a curve, why it moves fastest at the equinox, and the actual arithmetic that puts it on a map.

Time zone map

The boundaries wander because they follow borders rather than meridians — which is exactly what makes a map more useful than a list.

World time zone map

Thirty-eight offsets across twenty-six hours, and the most informative parts of the map are the places where the pattern breaks.

US time zone map

Four zones across the mainland, six in total, and the interesting parts are the states the boundary runs through and the two that opt out of the clock change.

Meetings across zones

The overlap window, computed to the minute — the question a converter cannot answer.

When are two cities both awake?

The overlap window, computed to the minute — including the half-hour zones and the daylight-saving edges that whole-hour arithmetic quietly gets wrong.

Time zone overlap: finding the hours you share

Overlap is a number you can compute, and once you have it the scheduling argument usually ends — because most of those arguments are really about a constraint nobody had measured.

Working across time zones

The problems are not really about clocks. They are about who absorbs the inconvenience, and whether anybody decided that on purpose.

The best hour to reach someone eight zones away

There is usually exactly one good slot, it is at the edge of somebody's day, and picking it deliberately beats rediscovering it every week.

What “EOD” means when your client is in another zone

It reads like a deadline and behaves like a guess. Here is why the phrase fails specifically, and the wording that replaces it.

Odd zones and edge cases

Half hours, quarter hours and single-zone countries — the cases cheap converters get wrong.

Half-hour and quarter-hour time zones

Not rounding errors, not bugs — the legal time of about a fifth of the world's population, and the fastest way to find out whether a converter is any good.

Why is India 30 minutes off?

Because a country wide enough for two time zones chose one, and put it in the middle — a deliberate compromise, not an accident.

Nepal is 45 minutes off, and that’s deliberate

UTC+5:45 — fifteen minutes from India, set to a mountain, and one of only three quarter-hour offsets in the world.

How many time zones does Russia have?

Eleven — more than any other country, spanning nine hours from Kaliningrad to Kamchatka, and the count has changed twice in the last fifteen years.

Which country celebrates New Year first?

Kiribati — and the celebrations run for twenty-five hours before the last inhabited place gets there, which is longer than a day.

Jet lag and travel

What crossing zones does to your body clock, and what the evidence says actually helps.

Jet lag: what crossing zones does to your clock

Your body keeps its own approximately-24-hour clock, and it does not move as fast as an aeroplane. The mismatch is the whole condition.

How to avoid jet lag

You cannot avoid it entirely, but light timing does most of the work — and getting the timing backwards makes things measurably worse.

How long does jet lag last?

About a day per time zone going west, and rather worse going east — with a long tail that catches people out on the return leg.

Hour-by-hour conversions

Every working hour across the common zone pairs, each computed with both halves of the year, the date shift and the next clock change that moves it — see the full set.

Situations, not queries

The guides answer the question. The scenarios cover the part that actually costs you something — the standup somebody always takes at 6am, the birthday wished a day late, the call that lands at 3am.

Time zones, answered plainly.