How to Convert Between Time Zones

Converting a time between two places is simple arithmetic, and people still get it wrong constantly — usually not because the sum is hard, but because something other than the sum changed. The date rolled over. One city had moved its clocks and the other hadn't. The offset turned out not to be a whole number of hours.

Everything below is built on one idea: every time zone is defined as an offset from UTC, so the reliable way to convert between two zones is to go through UTC rather than reasoning about them directly.

UTC, the Reference Point

Coordinated Universal Time is the standard every other zone is measured against. It doesn't observe daylight saving and doesn't change, which is exactly what makes it useful as a fixed point. When someone writes "UTC-5" or "GMT+2", they're telling you how many hours to add or subtract from UTC to get local time.

• It's maintained by a global network of atomic clocks, with leap seconds occasionally added to keep it aligned with the Earth's rotation
• It replaced GMT as the international standard in 1972, though GMT survives as the name of the UK's winter time zone
• A trailing "Z" — as in 14:30Z — means the time is given in UTC
• Most offsets are whole hours, but not all of them

The Arithmetic

To go from UTC to a local time, add that zone's offset:

So 3:00 PM UTC in Eastern Standard Time, which is UTC-5, is 3:00 PM minus five hours, or 10:00 AM.

To go directly between two zones, take the difference between their offsets:

Converting 2:00 PM in California (UTC-8) to Berlin (UTC+1): the difference is +1 minus -8, which is nine hours. 2:00 PM plus nine hours is 11:00 PM the same day.

The subtraction of a negative number is where most mental arithmetic fails. If you're going through it in your head, converting to UTC first and then out again is slower but much harder to get wrong.

Daylight Saving Changes the Answer

Offsets are not constant through the year. Roughly a third of the world shifts its clocks forward in summer, and the countries that do it don't all switch on the same date.

New York and London are normally five hours apart. The United States switches to daylight saving on the second Sunday in March; the EU and UK switch on the last Sunday. For those weeks in between, the two cities are four hours apart instead of five. The same thing happens in reverse in autumn. A recurring meeting set before the gap will quietly move for one side, and this is probably the single most common cause of a missed international call.

Southern hemisphere countries run the opposite way — their summer is the northern winter — so the difference between, say, London and Sydney changes by two hours across the year rather than one.

Where Conversions Go Wrong

The date changes and nobody notices

This is the most expensive mistake, because the time looks plausible. A 9:00 PM Tuesday meeting in San Francisco is 5:00 AM on Wednesday in London. Get the hour right and the day wrong and the invite is still wrong. Any conversion crossing more than about eight hours is worth checking the date on, and anything crossing the International Date Line always is.

AM and PM

12-hour clocks are ambiguous in exactly the place it matters: 12 AM is midnight and 12 PM is noon, which almost nobody recalls with confidence under time pressure. Writing times in 24-hour format removes the problem entirely, which is why aviation, medicine and the military all use it.

Not every offset is a whole hour

A number of zones are offset by thirty or forty-five minutes, and any conversion that assumes whole hours will be wrong for them:

• India — UTC+5:30, a single zone for the whole country
• Nepal — UTC+5:45, the only zone in the world on a 45-minute offset
• Central Australia — UTC+9:30, and UTC+10:30 during daylight saving
• Newfoundland — UTC-3:30
• Chatham Islands — UTC+12:45

Practical Habits

• Always name the zone. "3 PM" is not a time to someone in another country. Write "3 PM Eastern", or better, give it in UTC
• Use 24-hour format in anything written down
• Put a converter link in the invite so people can verify rather than trust your arithmetic
• Confirm important times explicitly — a one-line "that's 2 PM your time, correct?" catches errors before they cost anyone a morning
• Check the offset near DST transitions rather than relying on the difference you remember

Finding a Time That Works for Everyone

Across a wide enough spread there is no good time, only less bad ones. New York, London and Tokyo have no hour when all three are in normal working hours, so the question becomes who absorbs the inconvenience and how often.

• Rotate it. If the meeting recurs, move which region takes the awkward slot, so the cost is shared rather than always paid by the same person
• Find and protect the overlap you do have. Most teams have a narrow window where enough people are working; it's worth reserving that for discussions that genuinely need everyone, rather than status updates
• Or spread it evenly. 6 AM in California is 2 PM in London and 11 PM in Tokyo — nobody's ideal, but nobody is singled out either

The Short Version

The arithmetic is genuinely easy: convert to UTC, convert back out. What's hard is remembering that offsets move twice a year, that some of them aren't whole hours, and that the date can change underneath you. Those three things cause nearly every mistaken meeting invite.

In practice it's worth checking with something that tracks daylight saving for you rather than doing it from memory — the TimeZonder converter applies each location's current offset, including during the weeks when regions are out of step.