How Time Zones Work
Every zone is defined as an offset from Coordinated Universal Time (UTC). Converting between two zones means going through UTC: subtract the source offset, then add the destination offset.
UTC = Local time − Offset
Destination = UTC + Destination offset
The Earth turns 15° of longitude per hour, which is why zones are nominally 15° wide. In practice borders follow political lines, and some zones are enormous — China spans five geographic zones but observes one.
Major Offsets
| Offset | Cities | Note |
|---|---|---|
| UTC−8 / −7 | Los Angeles, Vancouver | Pacific; −7 in summer |
| UTC−6 / −5 | Chicago, Mexico City | Central |
| UTC−5 / −4 | New York, Toronto | Eastern; −4 in summer |
| UTC−3 | São Paulo, Buenos Aires | No DST since 2019 in Brazil |
| UTC+0 / +1 | London, Lisbon | +1 in summer (BST) |
| UTC+1 / +2 | Paris, Berlin, Madrid | Central European Time |
| UTC+3 | Istanbul, Moscow, Nairobi | Turkey and Russia no longer change clocks |
| UTC+5:30 | All of India | One zone for the whole country |
| UTC+8 | Singapore, Beijing, Perth | No DST |
| UTC+9 | Tokyo, Seoul | No DST |
| UTC+10 / +11 | Sydney, Melbourne | +11 in the southern summer |
The Awkward Offsets
Not every zone is a whole number of hours from UTC:
- UTC+5:30 — India and Sri Lanka
- UTC+5:45 — Nepal, deliberately set 15 minutes ahead of India
- UTC+9:30 — central Australia
- UTC+12:45 — the Chatham Islands
- UTC+14 — Kiribati's Line Islands, the first place to see each new day
The extremes matter: UTC−12 and UTC+14 are 26 hours apart, so at some moments three different calendar dates exist simultaneously on Earth.
Daylight Saving Time
Roughly 70 countries move their clocks seasonally, and they do not do it on the same dates. The United States switches on the second Sunday in March and the first Sunday in November; the European Union on the last Sundays of March and October. For several weeks each year the usual offset between New York and London is therefore four hours rather than five.
The southern hemisphere runs opposite: Sydney is on daylight time from October to April, which is when the northern hemisphere is not.
An increasing number of places have abandoned the practice — Russia in 2011, Turkey in 2016, Mexico in 2022, and most of Asia and Africa never adopted it. This calculator uses fixed offsets, so choose the summer or winter entry appropriate to your date.
Planning Meetings Across Zones
| Pair | Typical gap | Working hours shared |
|---|---|---|
| New York – London | 5 hours | 3 hours (afternoon in London) |
| London – Dubai | 4 hours | 4 hours |
| London – Singapore | 8 hours | 1 hour |
| New York – Tokyo | 13 hours | None — someone works outside hours |
| San Francisco – Sydney | 17 hours | None |
The chart above shows this directly. When the overlap is zero, the practical options are asynchronous work or accepting that one side takes an early or late call.
UTC, GMT and Zulu
UTC is the modern atomic time standard. GMT is a time zone based on the Greenwich meridian, and although the two are within a second of each other, only UTC is a standard — GMT is a zone that Britain uses in winter. Zulu time (Z) is the aviation and military name for UTC, from the NATO phonetic alphabet.
Frequently Asked Questions
Why does the date change when I convert?
Because the offset pushes the time past midnight. Converting 14:00 in New York to Tokyo lands at 03:00 the following day.
Does this account for daylight saving?
Not automatically. Choose the offset that applies on your date — for example UTC−4 for New York in summer and UTC−5 in winter.
What is the largest possible time difference?
26 hours, between UTC−12 and UTC+14. Both exist, so two inhabited places can be more than a full day apart.
Which countries have only one time zone despite being wide?
China spans about 60° of longitude on a single offset, and India uses one half-hour offset across the whole country. Both prioritise administrative simplicity over solar accuracy.