Time Difference Calculator

What a time difference measures

A time zone is a political agreement about what the clock says, not a line drawn on the ground. The Earth turns 360 degrees in 24 hours, which works out to 15 degrees of longitude per hour, so a town one hour east of another sees the sun rise one hour earlier. That part is physics. The rest is government: zones follow state and national borders, and they get redrawn when a legislature decides to redraw them.

Every zone is described by an offset from UTC, the clock at zero degrees longitude, which the NIST time and frequency division keeps and publishes. New York is UTC-5 in winter and UTC-4 in summer. London is UTC+0 in winter and UTC+1 in summer. Tokyo is UTC+9 all year. The gap between two places is one offset minus the other, so 0 minus -5 gives 5, and London sits 5 hours ahead of New York. Run that subtraction on the date you care about rather than on a printed chart, because the offsets move with the season.

Most offsets land on the hour. A few do not. India is UTC+5:30, Newfoundland is UTC-3:30 and Nepal is UTC+5:45, so a call to Bangalore can land on the half hour while a call to Tokyo lands on the hour.

Worked example: 9 a.m. in New York is 2 p.m. in London

Take November 10, 2026, a Tuesday in the middle of standard time on both sides of the Atlantic. New York is on EST at UTC-5 and London is on GMT at UTC+0. The gap is 5 hours.

• 9:00 a.m. in New York is 14:00 UTC.
• London adds nothing to UTC, so it is 2:00 p.m. there.
• Noon in New York is 5:00 p.m. in London, which is the last civilized hour for a call.
• 5:00 p.m. in New York is 10:00 p.m. in London, which is dinner, not a meeting.

Now look at the overlap instead of a single time. Your 9 to 5 day runs from 2:00 p.m. to 10:00 p.m. London time. Their 9 to 5 day runs from 4:00 a.m. to noon your time. The two blocks touch between 9:00 a.m. and noon your time, which is 2:00 p.m. to 5:00 p.m. theirs. That is a 3 hour window, and it is the only part of the day when both offices are open.

Book the meeting at 10:00 a.m. Eastern and it is 3:00 p.m. in London, with an hour of slack on each side. Book it at 8:00 a.m. Eastern and you are asking a London colleague to join at 1:00 p.m., which works, but the 5:00 p.m. end of their day is already gone.

Daylight saving time moves the gap twice a year

If both countries changed their clocks on the same weekend, the 5 hour gap would never move. They do not. The United States springs forward on the second Sunday in March and falls back on the first Sunday in November. The United Kingdom and the rest of Europe switch on the last Sunday in March and the last Sunday in October. In 2026 that means the US moves on March 8 and November 1, while the UK moves on March 29 and October 25.

Between March 8 and March 29, New York is already on daylight time while London is still on GMT. The gap drops to 4 hours, so 9:00 a.m. Eastern is 1:00 p.m. in London. The same thing happens from October 25 to November 1, when London has fallen back and New York has not. For those two stretches each year, every recurring meeting shifts by an hour unless someone updates the invite.

Southern hemisphere zones flip the pattern. Sydney is UTC+11 in January and UTC+10 in July, while New York goes from UTC-5 to UTC-4, so the gap between them swings from 16 hours in January to 14 hours in July. A 9:00 a.m. Tuesday call in New York is 1:00 a.m. Wednesday in Sydney in January, and 11:00 p.m. Tuesday in July.

Two US states sit out the whole exercise. Arizona keeps standard time all year, and Hawaii has never observed daylight saving. That is why Phoenix is 2 hours behind New York in winter and 3 hours behind in summer.

Common city pairs and the gap you get

The table below lists the gap in winter (January) and in summer (July), which covers most of the year in both hemispheres. A pair with the same number in both columns keeps its gap all year, because both cities shift together.

The two pairs that stay put are the ones where both cities follow the same rule on the same date. The pairs that move are the ones where one side observes daylight time and the other does not, or where the seasons are reversed.

Finding an hour that works for both sides

Put both workdays on one clock and the answer falls out. Convert your 9 to 5 into their time, convert their 9 to 5 into yours, and take the later start and the earlier end. If the end comes before the start, there is no shared window that day and no amount of scheduling will create one.

For New York and London that gives 9:00 a.m. to noon Eastern, a 3 hour block. For New York and Los Angeles it gives 9:00 a.m. to 5:00 p.m. Pacific, which is the entire day, so the constraint is courtesy rather than arithmetic. For New York and Sydney the two blocks never touch: their 9 to 5 is your 5:00 p.m. to 1:00 a.m., so someone takes a late call or the meeting becomes a document.

When the overlap is thin, three habits keep the calendar sane. Rotate the awkward slot so the same person is not always the one on a 6:00 a.m. call. Keep the live hour for decisions and send the status updates in writing. And leave 15 minutes of buffer at each edge, because a meeting that ends exactly at the top of the hour is a meeting that runs over into someone's dinner.

US zones, offsets and who skips the clock change

These are the offsets for the main US zones. The summer column applies from the second Sunday in March to the first Sunday in November, except where the last column says otherwise.

The continental states are one hour apart from each other, which makes the arithmetic easy until you cross into Arizona. A team spread across New York, Chicago, Denver and Los Angeles covers 3 hours from coast to coast, and the whole spread fits inside one workday if the meeting starts at 1:00 p.m. Eastern.

Why UTC is the safest middle step

Converting directly from one local zone to another works, but it hides the thing that moves. Converting through UTC makes the moving part visible. A wall clock time becomes an instant, the instant is stored once, and every display is a subtraction from that instant. Nothing in the middle drifts.

UTC also survives the awkward zones. India at UTC+5:30 and Nepal at UTC+5:45 are simple arithmetic against UTC and messy arithmetic between each other. The date line is the other trap: Samoa and Kiribati sit at UTC+13 and UTC+14, so a Monday morning meeting in Auckland is Sunday afternoon in Los Angeles, and a calendar invite that ignores the date change will land on the wrong day.

Software solves this with the IANA time zone database, the list behind every operating system and browser. The official US clock, the one that defines the offset for the Eastern zone, is published at time.gov. That database is not a fixed table of offsets. It is a record of political decisions, updated whenever a government changes its rules, which happened when Mexico dropped daylight time in 2022 and when Egypt reintroduced it in 2023. This calculator reads the same rules your device uses, which is why it agrees with your phone and with the calendar app that just sent the invite.

Nine a.m. Eastern, seen from six other cities

Here is one time, held fixed, converted into the places US teams call most. All of these are on November 10, 2026, when the US and Europe are both on standard time.

The Sydney row is the one people get wrong. It is not just late, it is the next day, so a recurring Tuesday meeting shows up on Wednesday for that team and the date on the agenda no longer matches. Check the day column before you check the hour.

The other common mistake is trusting a memory of a gap. Someone learns that London is 5 hours ahead in February, schedules a March 20 call on that basis, and misses it by an hour because the US moved its clocks and the UK had not. Confirm the offset for the date, not for the season you remember.