Understanding Time Zones, DST, and City Time Differences

A practical guide to time zones, UTC offsets, timezone abbreviations like PST and PDT, and how Daylight Saving Time shifts the clock.

1. How Time Zones Work

Time zones are regions of the world that share the same standard time. The global reference point is Coordinated Universal Time (UTC), which replaced Greenwich Mean Time (GMT) as the international standard in 1972. Every time zone is defined as a fixed offset from UTC. For example, New York is UTC−05:00 during winter (Eastern Standard Time) and UTC−04:00 during summer (Eastern Daylight Time). Tokyo is UTC+09:00 year-round because Japan does not observe Daylight Saving Time.

UTC offsets for selected cities — each offset represents the number of hours ahead of or behind UTC.

Time zones are not purely geographical — they are political decisions. China spans five geographical time zones but uses a single official time (UTC+08:00) nationwide. India uses UTC+05:30 even though the country stretches across two geographical zones. These choices are maintained in the IANA Time Zone Database (also called the tz database or zoneinfo), the authoritative source used by virtually all computers and smartphones worldwide. The IANA database is updated several times a year as countries change their time zone rules. Each entry uses a Region/City naming convention — for example, America/New_York or Asia/Kolkata — rather than abbreviations, because abbreviations can be ambiguous.

The IANA Time Zone Database is updated several times a year and is used by virtually all computers, phones, and operating systems worldwide.

2. Finding the Time Difference Between Cities

The time difference between two cities is the gap between their UTC offsets. If New York is at UTC−05:00 and London is at UTC+00:00, New York is 5 hours behind London. This calculation is straightforward when both cities keep the same offset year-round. However, the difference can change when one or both cities observe Daylight Saving Time. New York and London are normally 5 hours apart, but for a few weeks in spring and autumn — when the US and Europe switch on different dates — the gap shrinks to 4 hours.

The time difference between New York (UTC−05:00) and London (UTC+00:00) is 5 hours — but this changes during DST transition weeks.

Try it: New York → London →

When comparing cities across hemispheres — for example, New York and Sydney — the time difference can shift by up to 2 hours throughout the year, because one city gains an hour from DST while the other loses one.

Try it: New York → Sydney →

3. PST vs PDT and Other Timezone Abbreviations

Most time zones that observe Daylight Saving Time have two abbreviations — one for standard time and one for daylight time. Common pairs include: PST/PDT (Pacific, UTC−08/−07), EST/EDT (Eastern, UTC−05/−04), CET/CEST (Central Europe, UTC+01/+02), and GMT/BST (UK, UTC+00/+01). San Francisco uses PST from early November through early March, and PDT the rest of the year. If someone asks "what time zone is San Francisco in?" the correct answer depends on the date.

Common timezone pairs — the standard abbreviation is used in winter, the daylight abbreviation in summer.

Some abbreviations are ambiguous. CST can mean Central Standard Time (UTC−06 in the US), China Standard Time (UTC+08), or Cuba Standard Time (UTC−05). IST can mean India Standard Time (UTC+05:30), Irish Standard Time (UTC+01), or Israel Standard Time (UTC+02). This is why the IANA Time Zone Database uses unambiguous Region/City identifiers like America/Chicago or Asia/Shanghai.

Try it: San Francisco time zone →

4. How Daylight Saving Time Changes the Clock

Daylight Saving Time (DST) moves clocks forward by one hour in spring and back by one hour in autumn. In the United States and Canada, DST begins on the second Sunday of March and ends on the first Sunday of November. In the European Union, DST begins on the last Sunday of March and ends on the last Sunday of October.

DST observance varies worldwide — the US and EU switch on different dates, and many countries skip DST entirely.

Countries in the Southern Hemisphere that observe DST switch in the opposite direction: clocks go forward in September or October and back in March or April. Many countries do not observe DST at all, including most of Africa, most of Asia (China, Japan, India, South Korea), Russia (since 2014), Iceland, and the US states of Arizona and Hawaii.

During the weeks when the US has already switched but Europe has not (or vice versa), time differences between American and European cities are one hour different from their usual gap. Our converter handles this automatically — just set the date.

Try it: New York → London (check any date) →

5. Half-Hour and 45-Minute Offsets

While most time zones are offset from UTC by whole hours, several important exceptions exist. India (UTC+05:30) uses a single half-hour offset for over 1.4 billion people. Nepal (UTC+05:45) is one of only two places with a 45-minute offset — the other being the Chatham Islands of New Zealand (UTC+12:45).

$Diagram showing countries with fractional UTC offsets$

India (UTC+05:30), Nepal (UTC+05:45), Iran (UTC+03:30), and others use fractional UTC offsets.

Other half-hour offsets include Iran (UTC+03:30), Afghanistan (UTC+04:30), Myanmar (UTC+06:30), and parts of Australia (UTC+09:30). These fractional offsets mean the time difference between two cities is not always a round number. For example, London to Mumbai is 5 hours 30 minutes, and Mumbai to Kathmandu is just 15 minutes.

Try it: London → Mumbai (5h 30m) → Try it: Mumbai → Kathmandu (15m) →

Our converter displays fractional offsets naturally — for example, "5h 30m ahead" instead of "5.5 hours ahead" — so you always get a clear reading, even for unusual offsets like Nepal's UTC+05:45.

Try it: Compare all fractional offsets →

Sources

IANA Time Zone Database · Daylight Saving Time by Country (Wikipedia) · List of UTC Offsets (Wikipedia)

Ready to Convert Time Zones?

Select two or more cities and instantly see the time difference, current local times, timezone abbreviations, and working-hour overlaps — with automatic DST adjustments for any date.

Open Time Zone Converter