In the graph below you see the time zone difference between Greenwich Mean Time and Eastern Daylight Time. Drag the bubble to adjust the time and to see the corresponding time in the other time zone.
Greenwich Mean Time is the time zone of the 0 meridian, located in London, United Kingdom, including cities like London, Lisbon and Dublin (in the winter). Eastern Daylight Time is the time zone of the Canadian and US East Coast when observing daylight saving time in the summer, with places like Montreal, Ottawa, New York, Washington D.C. And Miami.
Usually it is preferable to schedule meetings after lunch for the person in GMT and the morning for the person in EDT.
GMT, or Greenwich Mean Time, is the zero when it comes to time zones – they are all counted from the zero meridian that runs through the village of Greenwich.
While both the UK and Portugal are officially on GMT you need to pay attention to whether they are actually observing it or are using their own. Britain in the summer for example use British Summer Time – which is different than GMT.
London, Lisbon, Dublin, Edinburgh
GMT does not observe DST as it is the zero hour time zone – it never changes.
This is the summer time of the Eastern US, Canada and parts of the Caribbean. It is in effect between March and October for most locations and is four hours behind UTC. It is important to note that even within certain states, e.g. Florida, Michigan, and Tennessee there are locations that do NOT observe EST/EDT but instead belong to other time zones. The same goes for the Canadian provinces Ontario and Quebec.
Most of the Eastern seaboard of the US observe Eastern Daylight Time, as well as most locations in Eastern Canada although there are a few exceptions. Locations such as New York, Washington, Quebec and Ontario are all on Eastern Standard Time. Also some Caribbean locations such as Jamaica, Haiti and Panama are in this time zone.
With the exception of Panama, Jamaica and a couple of locations in Canada and the US daylight saving time is observed. Unless you are sure that the location you have in mind is truly in EST/EDT it is always better to use the exact location name rather than the time zone abbreviation.