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Time zones8 min read

How to Convert Time Between Time Zones, Step by Step (With Examples)

Convert any time zone with one rule: subtract the starting offset, add the target offset. Worked examples cover midnight, half-hour zones and daylight saving.

By Timelive Editorial TeamPublished How we check facts

On this page
  1. The rule: subtract the source offset, add the target offset
  2. Three worked examples
  3. The daylight saving trap: the difference is not a constant
  4. Half-hour and quarter-hour offsets
  5. Quick reference: common differences
  6. A checklist for converting a meeting time
  7. Frequently asked questions
  8. Sources and further reading

The arithmetic of a time zone conversion is simple. What goes wrong is the input: an offset that was true last month and is not true on the day of the meeting.

The rule: subtract the source offset, add the target offset#

Every clock reads UTC plus or minus a fixed amount, its UTC offset. UTC is the shared reference and does not follow the seasons (UTC vs GMT: What Is the Difference and Which Should You Use?). New York in winter is UTC-5, London in winter is UTC+0, and Kolkata is UTC+5:30 all year. A higher offset means a clock further ahead.

  1. Pick the date in the place where the time was set, because offsets depend on it.
  2. Look up both offsets for that date, using the city rather than a zone label.
  3. Subtract: difference = target offset - source offset.
  4. Add the difference to the source time, hours and minutes separately.
  5. Fix the date. At 24:00 or later, subtract 24 hours and go one day forward. Below 00:00, add 24 hours and go one day back.

Converting through UTC (subtract the source offset, then add the target offset) gives the same answer and is safer with three or more cities, because every city is compared with one clock.

Three worked examples#

All three use October 2026 dates.

New York to London on an ordinary day#

You have 15:00 (3:00 pm) on Tuesday, October 13, 2026, in New York, on Eastern Daylight Time (UTC-4). London is on British Summer Time (UTC+1).

Difference: (+1) - (-4) = +5 hours. Then 15:00 + 5:00 = 20:00. It is 8:00 pm in London, the same day.

Los Angeles to Kolkata: forward across midnight#

You have 20:00 on Tuesday, October 13, 2026, in Los Angeles, on Pacific Daylight Time (UTC-7). Kolkata is UTC+5:30.

Difference: (+5:30) - (-7:00) = +12:30. Then 20:00 + 12:30 = 32:30, which is past 24:00, so subtract 24 hours: 08:30 on Wednesday, October 14. EST to IST: Time Difference, Conversion Chart and Call Times covers the US East Coast to India pair in more detail.

Tokyo to New York: back across midnight#

You have 09:00 on Wednesday, October 14, 2026, in Tokyo (UTC+9). New York is UTC-4.

Difference: (-4) - (+9) = -13 hours. Then 09:00 - 13:00 = -04:00, which is below zero, so add 24 hours: 20:00 on Tuesday, October 13.

The daylight saving trap: the difference is not a constant#

Many countries move their clocks forward an hour in the warm months, and their offset moves with them: New York is UTC-5 in winter and UTC-4 in summer. So the difference between two places changes when either one changes, and often only one has changed yet (How Daylight Saving Time Works, Why It Exists and Who Still Uses It has the mechanism).

The dates are set by law, and they differ. The United States changes at 2:00 a.m. on the second Sunday in March and the first Sunday in November (15 U.S.C. § 260a, as summarized by the Congressional Research Service). The European Union and the UK change on the last Sundays of March and October (Directive 2000/84/EC). The US therefore moves first in spring and Europe moves first in autumn, and for a few weeks London is 4 hours ahead of New York, not 5:

  • 2026: Sunday, March 8 to Sunday, March 29 (only the US has changed), and Sunday, October 25 to Sunday, November 1 (only the UK has changed).
  • 2027: Sunday, March 14 to Sunday, March 28, and Sunday, October 31 to Sunday, November 7.

These dates are computed from the IANA time zone database, with rules as of October 2026. When Do the Clocks Change? Daylight Saving Dates for 2026 and 2027 lists the dates for other regions.

On Tuesday, October 27, 2026, 15:00 in New York (UTC-4) is not 20:00 in London. London went back to GMT on October 25, so the difference is (0) - (-4) = +4 and the answer is 19:00. Two weeks earlier, on October 13, it was 20:00.

Other places behave differently:

  • Places that never change still shift against New York because New York does. India, Japan and Dubai had no offset change in 2026 or 2027, yet Kolkata is 10 h 30 min ahead of New York in mid-January 2027 and 9 h 30 min ahead in mid-July.
  • Southern Hemisphere places can run the other way. Sydney's daylight time falls in the northern winter: its clocks went back to UTC+10 on April 5, 2026, and forward to UTC+11 on October 4, 2026.

On change-over night a local time can fail to exist or happen twice: 2:30 am on March 8, 2026, never happened in New York, and 1:30 am on November 1, 2026, happened twice. The Timelive converter moves a time that does not exist forward by an hour and uses the first of a repeated time. Avoid those hours for a real meeting.

Half-hour and quarter-hour offsets#

Most zones sit a whole number of hours from UTC, but not all. India is UTC+5:30 with no daylight time, and Newfoundland, Canada, is UTC-3:30 in winter and UTC-2:30 on daylight time, according to timeanddate.com. Nepal is UTC+5:45 (from the IANA time zone database, with no daylight time in 2026 or 2027). Why India Is UTC+5:30: Half-Hour and 45-Minute Time Zones explains why. The method does not change: handle hours and minutes separately.

  • London at 09:00 on January 15, 2027 (UTC+0) to Kolkata: +5:30, so 14:30.
  • The same London time to Kathmandu: +5:45, so 14:45.

An easy slip is to write 5:30 as 5.3 hours, which is 5 hours 18 minutes, 12 minutes short. Half an hour is 0.5.

Quick reference: common differences#

PlaceCompared withMid-January 2027Mid-July 2027Who changes clocks
Londonahead of New York5 h5 hBoth, on different dates
Los Angelesbehind New York3 h3 hBoth, on the same dates
Kolkataahead of New York10 h 30 min9 h 30 minOnly New York
Kolkataahead of London5 h 30 min4 h 30 minOnly London
Tokyoahead of New York14 h13 hOnly New York
Sydneyahead of London11 h9 hBoth, in opposite seasons

Values are for January 15 and July 15, 2027, computed from the IANA database with rules as of October 2026. Between those dates a pair can differ: New York to London is 4 hours in the windows above. Rules change, so for a meeting months away, check again closer to the date. Where the exact time has legal force, use the official source.

A checklist for converting a meeting time#

  1. Write down the date and the city where the time was set, not only a zone label.
  2. Look up both UTC offsets for the meeting date, not for today.
  3. Subtract, add, and fix the date. Say the date for each city.
  4. Check the direction: the place with the higher offset has the later clock.
  5. Put every city and the UTC time in the invitation: "Tuesday, October 13, 15:00 New York, 20:00 London, 00:30 Wednesday, October 14, Kolkata (19:00 UTC)." Check it in a converter before you send it.

Frequently asked questions

How do I convert time from one time zone to another?

Find the UTC offset of both places on the date of the event, subtract the starting offset from the target offset, and add the result to your time. On January 15, 2027, London is UTC+0 and Kolkata is UTC+5:30, so 9:00 am in London is 2:30 pm in Kolkata.

If the answer reaches 24:00 or goes below 00:00, shift the date by one day.

What is the time difference between New York and London?

London is 5 hours ahead of New York for most of the year, and 4 hours ahead when one city has changed its clocks and the other has not. The US changes on the second Sunday in March and the first Sunday in November; the UK changes on the last Sundays of March and October.

In 2026 the 4-hour gap runs from March 8 to 29 and from October 25 to November 1. In 2027 it runs from March 14 to 28 and from October 31 to November 7.

How do I tell whether the converted time is the next day or the previous day?

Do the sum before you reduce it. A result of 24:00 or more is the next day: 8:00 pm in Los Angeles plus 12 h 30 min is 32:30, which is 8:30 am the following day in Kolkata. A negative result is the previous day: 9:00 am in Tokyo minus 13 hours is -04:00, which is 8:00 pm the day before in New York.

How do I convert times with half-hour offsets like India's?

Keep hours and minutes separate and add both. India is UTC+5:30, and Toronto is UTC-5 in January, so the difference is (+5:30) - (-5:00) = +10:30 and 10:15 in Toronto becomes 20:45 in Kolkata. Do not write 5:30 as 5.3 hours, which is 5 hours 18 minutes; the decimal form is 5.5.

Sources and further reading

  1. Daylight Saving Time (DST) — Congressional Research Service
  2. 15 U.S.C. 260a, Advancement of time or changeover dates — U.S. Government Publishing Office
  3. Directive 2000/84/EC on summer-time arrangements — EUR-Lex, Publications Office of the European Union
  4. Half Hour and 45-Minute Time Zones — timeanddate.com

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