Whether you're scheduling international calls, catching live events, or coordinating with overseas colleagues, understanding time zones is essential. This guide explains how time zones work and helps you convert times accurately.
UK Time Zones
GMT (Greenwich Mean Time): UK winter time (late October to late March) - UTC+0
BST (British Summer Time): UK summer time (late March to late October) - UTC+1
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Major Time Zone Offsets from UK
Hours difference when UK is on GMT (winter):
New York (EST)
Los Angeles (PST)
Dubai (GST)
Mumbai (IST)
Singapore (SGT)
Sydney (AEDT)
Complete Time Zone Reference
| City | Time Zone | UTC Offset | Difference from UK (GMT) |
|---|---|---|---|
| London (Winter) | GMT | UTC+0 | — |
| London (Summer) | BST | UTC+1 | — |
| Paris, Berlin | CET/CEST | UTC+1/+2 | +1 hour |
| Moscow | MSK | UTC+3 | +3 hours |
| Dubai | GST | UTC+4 | +4 hours |
| Mumbai | IST | UTC+5:30 | +5:30 hours |
| Bangkok | ICT | UTC+7 | +7 hours |
| Hong Kong, Singapore | HKT/SGT | UTC+8 | +8 hours |
| Tokyo | JST | UTC+9 | +9 hours |
| Sydney (Summer) | AEDT | UTC+11 | +11 hours |
| Sydney (Winter) | AEST | UTC+10 | +10 hours |
| Auckland | NZDT/NZST | UTC+13/+12 | +12/+13 hours |
| New York (Winter) | EST | UTC-5 | -5 hours |
| New York (Summer) | EDT | UTC-4 | -4 hours |
| Chicago | CST/CDT | UTC-6/-5 | -6/-5 hours |
| Denver | MST/MDT | UTC-7/-6 | -7/-6 hours |
| Los Angeles | PST/PDT | UTC-8/-7 | -8/-7 hours |
How to Convert Times
Example 1: UK to New York
Question: What time is 2pm London in New York?
Winter (GMT to EST): 2pm - 5 hours = 9am
Summer (BST to EDT): 2pm - 5 hours = 9am (difference stays the same when both use DST)
Example 2: Sydney to UK
Question: If it's 8pm in Sydney (AEDT), what time is it in London?
Calculation: 8pm - 11 hours = 9am (same day)
Example 3: Meeting Across Zones
Question: Schedule a meeting that works for London, New York, and Los Angeles
If London = 5pm:
- New York: 12pm (noon)
- Los Angeles: 9am
Best window: 2pm-6pm London time works for all three during business hours.
Daylight Saving Time
Daylight saving complicates time zone calculations because countries change clocks on different dates:
| Region | Clocks Forward | Clocks Back |
|---|---|---|
| UK (BST) | Last Sunday of March | Last Sunday of October |
| USA/Canada | Second Sunday of March | First Sunday of November |
| EU (until 2021) | Last Sunday of March | Last Sunday of October |
| Australia (most states) | First Sunday of October | First Sunday of April |
Countries Without Daylight Saving
These major locations keep the same time year-round:
- Dubai, UAE (GST - UTC+4)
- Singapore (SGT - UTC+8)
- Hong Kong (HKT - UTC+8)
- Japan (JST - UTC+9)
- China (CST - UTC+8)
- India (IST - UTC+5:30)
- Most of Africa
- Most of South America
The International Date Line
The date line runs roughly through the Pacific Ocean at 180° longitude. Crossing westward adds a day; crossing eastward subtracts a day.
Date Line Example
Scenario: It's 11pm Saturday in London
Sydney (+11 hours): 10am Sunday
Los Angeles (-8 hours): 3pm Saturday
Australia is already in "tomorrow" relative to London!
Common Time Zone Abbreviations
| Abbreviation | Full Name | Region |
|---|---|---|
| GMT | Greenwich Mean Time | UK Winter |
| BST | British Summer Time | UK Summer |
| UTC | Coordinated Universal Time | Global Standard |
| EST/EDT | Eastern Standard/Daylight Time | US East Coast |
| CST/CDT | Central Standard/Daylight Time | US Midwest |
| PST/PDT | Pacific Standard/Daylight Time | US West Coast |
| CET/CEST | Central European Time/Summer | Europe |
| AEST/AEDT | Australian Eastern Standard/Daylight | Sydney |
| IST | India Standard Time | India |
| JST | Japan Standard Time | Japan |
Working Across Time Zones
Finding Meeting Times
Ideal meeting windows for common combinations:
| Locations | Best UK Time | Why It Works |
|---|---|---|
| UK + New York | 2pm - 6pm | 9am-1pm NY (business hours) |
| UK + Los Angeles | 4pm - 7pm | 8am-11am LA (morning there) |
| UK + India | 9am - 1pm | 2:30pm-6:30pm India (afternoon) |
| UK + Australia | 7am - 9am | 5pm-7pm Sydney (evening) |
Quick Reference: When It's Noon in London
| City | Time (GMT) | Time (BST) |
|---|---|---|
| New York | 7am | 7am |
| Los Angeles | 4am | 4am |
| Paris/Berlin | 1pm | 1pm |
| Dubai | 4pm | 3pm |
| Mumbai | 5:30pm | 4:30pm |
| Singapore | 8pm | 7pm |
| Tokyo | 9pm | 8pm |
| Sydney | 11pm* | 9pm* |
*Sydney times vary based on Australian DST
How Time Zones Work
Time zones are based on the Earth's rotation: the planet completes one full 360-degree rotation every 24 hours, meaning it rotates 15 degrees per hour. In theory, this creates 24 neat time zones of 15 degrees longitude each, all referenced to the Prime Meridian (0 degrees longitude) at Greenwich, London. In practice, time zone boundaries follow national and regional borders rather than straight longitudinal lines, creating a far more complex map.
Coordinated Universal Time (UTC) replaced Greenwich Mean Time (GMT) as the global standard in 1972, though the two are functionally identical for everyday purposes. UTC is maintained by a network of over 400 atomic clocks worldwide and is adjusted by leap seconds when necessary to stay synchronised with the Earth's slightly irregular rotation. Each time zone is expressed as an offset from UTC — for example, UTC+1 for Central European Time or UTC-5 for US Eastern Standard Time.
Target time = Source time - Source UTC offset + Target UTC offset
Example (London to Tokyo):
Tokyo time = London time - (UTC+0) + (UTC+9) = London time + 9 hours
Not all time zones differ by whole hours. India uses UTC+5:30, Nepal uses UTC+5:45, and the Chatham Islands of New Zealand use UTC+12:45. These half-hour and 45-minute offsets exist because countries chose offsets that best represent their geographical position relative to the sun. Australia has three time zones, one of which (Australian Central Standard Time) is UTC+9:30. These unusual offsets are important to account for when scheduling international meetings or setting up automated processes.
UK Time Zone Rules and Daylight Saving
The United Kingdom uses GMT (UTC+0) during winter and British Summer Time (BST, UTC+1) from the last Sunday in March to the last Sunday in October. This single-hour change affects every international time calculation for approximately seven months of the year. British Summer Time was first introduced during World War I through the Summer Time Act 1916, intended to save energy by making better use of daylight hours.
The clocks change at 1:00 AM GMT in spring (moving forward to 2:00 AM BST) and at 2:00 AM BST in autumn (moving back to 1:00 AM GMT). This creates a particular challenge for businesses: the spring change effectively loses an hour, while the autumn change repeats one hour. Automated systems, scheduled tasks, and international meeting times all need careful management around these transitions. For 2025, the clocks go forward on 30 March and back on 26 October.
The question of whether the UK should adopt permanent summer time (BST year-round) or even Central European Time (CET, UTC+1) is periodically debated. Proponents argue it would reduce road accidents in winter evenings, boost tourism revenue, and align UK business hours more closely with European partners. However, Scotland and Northern Ireland, being further north and west, would experience very late sunrises — as late as 10:00 AM in northern Scotland during December — making permanent BST unpopular in those regions. The most recent parliamentary attempt, the Daylight Saving Bill 2010-12, passed its second reading but was ultimately withdrawn.
Frequently Asked Questions About Time Zones
Why Do Some Countries Change Their Clocks and Others Do Not?
Daylight saving time (DST) is primarily observed in countries at higher latitudes where seasonal daylight variation is significant. Approximately 70 countries use DST, mostly in Europe and North America. Countries near the equator — such as those in most of Africa, South America, and South-East Asia — have relatively consistent daylight hours year-round, making DST unnecessary. Some countries have tried DST and abandoned it: Russia permanently moved to UTC+3 in 2014, and Turkey adopted permanent summer time (UTC+3) in 2016. The EU proposed ending mandatory clock changes in 2019, but member states have not yet agreed on whether to adopt permanent summer or winter time.
How Do I Calculate the Time Difference When Both Countries Use DST?
The complication arises because countries change their clocks on different dates. The UK and most EU countries share the same DST changeover dates, so the time difference between London and Paris is always 1 hour. However, the US changes clocks 2-3 weeks earlier in spring and 1 week later in autumn than the UK. During these transitional periods, the London-New York difference is 4 hours instead of the usual 5 (or 6 in winter). Australia's DST runs from October to April (opposite to the Northern Hemisphere), meaning the London-Sydney difference can be 9, 10, or 11 hours depending on the time of year and which country is currently observing DST.
What Is the Best Time to Schedule a Meeting Across UK, US, and Asia?
Finding overlap between UK, US East Coast, and East Asian time zones is notoriously difficult. When it is 9:00 AM in London, it is 4:00 AM in New York and 6:00 PM in Tokyo — no single hour falls within standard business hours for all three. The closest practical compromise is 8:00-9:00 AM UK time (3:00-4:00 AM US East / 5:00-6:00 PM Tokyo) or 5:00-6:00 PM UK time (12:00-1:00 PM US East / 2:00-3:00 AM Tokyo). Most multinational companies solve this by rotating meeting times so no single region consistently bears the inconvenience, or by splitting meetings into two sessions — one for UK/US overlap and one for UK/Asia overlap.
UK Time Zones and British Summer Time
The United Kingdom operates on Greenwich Mean Time (GMT, also designated UTC+0) during winter months and British Summer Time (BST, UTC+1) during summer. The transition dates are standardised across the EU and UK: clocks spring forward one hour at 1:00 AM on the last Sunday of March, and fall back one hour at 2:00 AM on the last Sunday of October. These changes affect time zone conversions for approximately seven months of the year, and failing to account for BST when scheduling international calls or meetings is one of the most common time zone mistakes made by UK-based workers.
Greenwich Mean Time takes its name from the Royal Observatory in Greenwich, London, which was established in 1675 and became the reference point for global timekeeping. The Prime Meridian (0 degrees longitude) passes through the observatory, and GMT was adopted as the world's time standard at the International Meridian Conference in Washington in 1884. Today, the more precise Coordinated Universal Time (UTC) has replaced GMT as the international standard, but for practical purposes in the UK, GMT and UTC+0 are identical. The Royal Observatory remains a popular tourist destination and a UNESCO World Heritage Site, attracting visitors who can stand on the Prime Meridian line.
UK businesses operating internationally must navigate a complex web of time zone rules. The European Union observes summer time on the same dates as the UK, but proposals to abolish the biannual clock change have been debated in the European Parliament since 2019. If EU member states eventually adopt permanent summer or winter time, the time difference between the UK and its European trading partners could change depending on the season and each country's choice. The United States changes clocks on different dates (the second Sunday of March and the first Sunday of November), creating a period of two to three weeks each spring and autumn when the UK-US time difference shifts by one hour from its usual offset.
Practical Tips for Time Zone Conversions
- Always specify GMT or BST: When arranging meetings with international contacts, state whether the UK time you are quoting is GMT or BST to avoid confusion during the months near the clock change.
- Remember the US clock change differs: The US and UK change clocks on different weekends, creating brief periods where the usual time difference is off by one hour. Double-check conversions during March and November.
- Use UTC for unambiguous scheduling: When coordinating across multiple time zones, stating times in UTC eliminates confusion about daylight saving adjustments and is the standard used in aviation, computing, and international business.
More Questions About Time Zone Conversions
Does the whole of the UK use the same time zone?
Has the UK ever considered staying on BST all year round?
What time zone does the UK use for flight schedules?
James Mitchell, ACCA
Chartered Accountant & Former HMRC Advisor
James is a Chartered Certified Accountant (ACCA) specialising in UK personal taxation and financial planning. With over 12 years in practice and a background as a former HMRC compliance officer, he brings authoritative insight to complex tax topics.