In the graph below you see the time zone difference between Greenwich Mean Time and Mountain 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). Mountain Daylight Time is the time zone of the US and Canadian states around the Rocky Mountains, with places like Edmonton, Calgary, Denver and Salt Lake City.
Usually it is preferable to schedule meetings in the afternoon for the person in GMT and before lunch for the person in MDT.
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.
MDT is observed in the Western part of central US and Canada, as well as the North West corner of Mexico. It is 6 hours behind UTC and turns into MST in the summer. In Mexico, this time zone is known as Zona Pacifico (Pacific Zone). The name Mountain Daylight Time comes from it covering the Rocky Mountains in Western United States.
In the US, Arizona, Colorado, Montana, Utah, New Mexico and Wyoming are all observing Mountain Daylight Time but there is also a bunch where part of the state observe MDT and part another time zone, like CDT or PDT.
MDT is observed in parts of Canada, the US and Mexico. The only province in Canada to be fully covered by MDT is Alberta whereas other provinces like British Columbia and Northwest Territories are split between MDT and PDT.
In Mexico, it is only the states in the north western corner of the country, including Baja California, Sinaloa and Chihuahua that observe MDT.
The largest city observing Mountain Daylight Time is Phoenix, followed by Denver and El Paso/Juarez.
MDT is observed in almost all locations that observe Mountain Time, except the states of Arizona (US) and Sinaloa (Mexico). DST is in effect between March and November and adds one hour time difference to UTC.