How It Works

Converting between Eastern Time (US East Coast) and Greenwich Mean Time (UK) requires understanding both the base offset and how Daylight Saving Time affects each zone differently. GMT is the baseline time zone at 0 degrees longitude, while EST is 5 hours behind GMT.

The tricky part is that the US and UK observe Daylight Saving Time on different schedules. The US switches to EDT (Eastern Daylight Time) in mid-March and back to EST in early November. The UK switches to BST (British Summer Time) in late March and back to GMT in late October. This creates periods where the offset is different than expected.

When Do I Need This Converter?

This converter is essential for anyone coordinating across the Atlantic. Here are the most common situations:

International business calls: If your New York office needs to call London, timing is everything. A 9am meeting in New York hits London at 2pm (5-hour difference during winter) or 1pm (4-hour difference during parts of spring and fall).

Stock market trading: The London Stock Exchange opens at 8am GMT, which is 3am EST. The New York Stock Exchange opens at 9:30am EST, which is 2:30pm GMT in winter or 1:30pm BST in summer.

Flight bookings and arrivals: Airlines show times in local zones. A flight departing London at 10am GMT arrives in New York around 1pm EST (after an 8-hour flight), but you experience a time shift that makes it feel like only 3 hours passed.

Live events and broadcasts: A Premier League match kicking off at 3pm GMT starts at 10am EST in winter. Awards shows airing at 8pm EST start at 1am GMT the next day in winter.

What Makes EST to GMT Conversion Confusing?

The challenge with EST/GMT conversion isn’t the base math. It’s that both regions change their clocks, but not on the same dates. This creates four distinct periods each year with different offsets.

From early January to mid-March: EST vs GMT, 5-hour difference. Both zones are in standard time.

From mid-March to late March: EDT vs GMT, 4-hour difference. The US has switched to daylight time but the UK hasn’t yet.

From late March to late October: EDT vs BST, 5-hour difference. Both zones are in daylight time.

From late October to early November: EDT vs GMT, 4-hour difference. The UK has switched back but the US hasn’t yet.

From early November to December: EST vs GMT, 5-hour difference. Both zones are back in standard time.

This is why you can’t just memorize “5 hours” and call it a day. The offset changes depending on the calendar, and most people don’t track these transitions.

Where Is EST Used?

Eastern Time covers the US East Coast and parts of the Midwest. Major cities include New York, Boston, Philadelphia, Washington DC, Atlanta, Miami, and Detroit. It’s also used in Toronto, Ottawa, and parts of Quebec in Canada.

About 47% of the US population lives in the Eastern Time zone, making it the most populated time zone in the country. When Americans say “Eastern Time” without context, they mean this zone.

Where Is GMT Used?

GMT is the time at the Royal Observatory in Greenwich, London. It’s the baseline for all time zones worldwide (though technically UTC has replaced GMT for scientific purposes, they’re functionally identical).

The UK, Ireland, Portugal, and Iceland use GMT in winter. During summer, the UK and Ireland switch to BST (British Summer Time), which is GMT+1. This summer shift is what creates the complexity in EST/GMT conversions.

Why Does the Time Difference Change?

The time difference changes because the US and UK don’t coordinate their Daylight Saving Time transitions. Each country sets its own dates based on local preferences and legislation.

The US changes clocks on the second Sunday in March (spring forward) and the first Sunday in November (fall back). The UK changes clocks on the last Sunday in March (spring forward) and the last Sunday in October (fall back).

The gap between these dates creates the confusion. In March, there’s typically a week or two where the US is on daylight time but the UK isn’t. In October/November, there’s a week or two where the UK has switched back but the US hasn’t.

How Do I Avoid Common Mistakes?

The biggest mistake is assuming the offset is always 5 hours. It usually is, but for several weeks each year, it’s only 4 hours. Always check the specific date when scheduling important calls or meetings.

Another error is confusing GMT with UK local time. During summer (late March through late October), the UK uses BST, not GMT. If someone in London says “let’s meet at 2pm my time” in July, they mean 2pm BST, which is actually 1pm GMT.

Calendar apps help but can still create confusion. If you create a meeting in Google Calendar set to EST and invite UK colleagues, they’ll see it converted to their local time. But if you just write “3pm EST” in an email, they need to manually calculate whether that’s 8pm or 7pm their time, depending on the season.

What About Meetings During the Transition Weeks?

The transition weeks (mid to late March, and late October to early November) are when mistakes happen most often. During these periods, double-check your conversions. The offset is temporarily 4 hours instead of the usual 5.

If you’re scheduling recurring meetings, be extra careful. A meeting that works at 2pm EST / 7pm GMT in January becomes 2pm EDT / 6pm GMT in April (once both zones shift), but might be 2pm EDT / 7pm BST for a week in late March when the UK is still on GMT.

Quick Reference Table

Eastern Time (US)	GMT/BST (UK)	Typical Offset	Period
9:00 AM EST	2:00 PM GMT	5 hours	Jan-mid Mar
9:00 AM EDT	1:00 PM GMT	4 hours	Mid-late Mar
9:00 AM EDT	2:00 PM BST	5 hours	Late Mar-late Oct
9:00 AM EDT	1:00 PM GMT	4 hours	Late Oct-early Nov
9:00 AM EST	2:00 PM GMT	5 hours	Early Nov-Dec
12:00 PM EST	5:00 PM GMT	5 hours	Winter overlap
3:00 PM EST	8:00 PM GMT	5 hours	End of US workday
6:00 PM EST	11:00 PM GMT	5 hours	US evening/UK late night

Frequently Asked Questions

Is GMT always 5 hours ahead of EST?

Usually, but not always. The standard offset is 5 hours when both zones are in standard time or both are in daylight time. But for a few weeks in March and again in October/November, the offset temporarily becomes 4 hours because the US and UK change their clocks on different dates.

What is the difference between GMT and UTC?

For practical purposes, GMT and UTC are the same. UTC (Coordinated Universal Time) is the scientific standard, while GMT (Greenwich Mean Time) is the traditional term. Both refer to the time at 0 degrees longitude. Most people still say GMT, but technically UTC is more accurate.

Does the UK use GMT year-round?

No. The UK uses GMT from late October to late March (winter), but switches to BST (British Summer Time, which is GMT+1) from late March to late October (summer). This is similar to how the US switches between EST and EDT, but on different dates.

What time should I schedule a call for both time zones?

The sweet spot for transatlantic calls is usually 1pm to 4pm EST (6pm to 9pm GMT in winter, or 5pm to 8pm BST in summer). This catches the end of the US workday and early evening in the UK. Anything earlier hits UK colleagues too late, anything later catches US workers after hours.

How do I handle midnight conversions?

When it’s midnight (12:00 AM) in New York, it’s 5:00 AM the same day in London during winter. This means anything happening after midnight EST is already well into the morning GMT. For events that cross midnight, pay careful attention to the date as well as the time.

Which time zone should I use for international contracts?

For legal documents between US and UK parties, specify the exact time zone (EST, EDT, GMT, or BST) along with the date. Better yet, include both times: “5:00 PM EST (10:00 PM GMT)” to eliminate confusion. Many contracts use UTC to avoid any ambiguity.

What happens during the spring and fall transitions?

During the transition periods (mid to late March, and late October to early November), the time difference temporarily becomes 4 hours instead of 5. This is the most dangerous time for scheduling errors. Always double-check conversions during these weeks.