Convert Greenwich Mean Time to Atlantic Standard Time (GMT to AST)

Understanding GMT and AST Time Zones

Greenwich Mean Time (GMT)

Greenwich Mean Time is observed in undefined and uses the offset UTC+0. Major cities in this time zone include London, Dublin, Lisbon. This time zone covers parts of Europe and/or Africa, home to major financial and cultural centers.

Atlantic Standard Time (AST)

Atlantic Standard Time operates at UTC-4, covering undefined. Key cities include Halifax, San Juan. The 4-hour backward offset from GMT means that when professionals in London start their workday at 9:00 AM, it is already 5:00 AM in Halifax.

GMT is also commonly referred to as London time, UK time, British time, England time. AST is also known as Atlantic Time, Puerto Rico time. So if you are searching for London time to Atlantic Time, this is the same conversion as GMT to AST.

Complete GMT to AST Conversion Table

The table below shows every two-hour increment converted from Greenwich Mean Time to Atlantic Standard Time. Business hours (9 AM – 5 PM) are highlighted for easy scheduling reference.

Times shown in 12-hour and 24-hour format. Last updated 2026-03-09.

Scheduling Between GMT and AST

Freelancers and contractors working across GMT and AST should clearly state their invoicing time zone. A 4-hour difference means an "end of business day" deliverable could mean very different things to each party.

The moderate 4-hour gap between GMT and AST means that there is a partial overlap of business hours. Teams can typically find a 3–4 hour window that works for both sides without requiring anyone to work outside normal hours.

Travel Between London and Halifax

For layover planning between undefined and undefined destinations, connecting flights often involve intermediate time zones. Use our converter to verify each leg independently to avoid missed connections.

Medical experts recommend allowing approximately one day of adjustment per hour of time difference. For the 4-hour shift between GMT and AST, plan for about 4 days to fully acclimate.

Daylight Saving Time Considerations

Some regions within a time zone do not observe DST. Arizona (MST) and Hawaii (HST) stay on standard time year-round, which can create unexpected offsets with neighboring states during summer months.

For the GMT to AST conversion specifically: GMT (UTC+0) observes DST, while AST (UTC-4) does not observe DST. This means the offset between them changes by one hour seasonally — the usual 4-hour gap becomes 3 or 5 hours during DST transitions.

Related Time Zone Conversions

References & Resources

• TimeAndDate.com — World Time Zones
• NIST — Official US Time Standard
• IANA Time Zone Database
• Wikipedia — Time Zone Abbreviations
• WorldTimeServer — Current Times Worldwide
• Royal Observatory Greenwich

Frequently Asked Questions