In the graph below you see the time zone difference between Greenwich Mean Time and British Summer 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). British Summer Time is the time zone of the United Kingdom when observing daylight saving time in the summer, with places like London, Dublin and Glasgow.
Usually it is preferable to schedule meetings in the morning for the person in GMT and after lunch for the person in BST.
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.
British Summer Time, or sometimes called British Standard Time, is the timezone of the United Kingdom in the summer months. It does not have a typical counterpart like most summer/winter pairs but is an exception to adjust for the dark winters in the UK!
United Kingdom only, including the Isle Of Man
All cities in the United Kingdom (England, Scotland, Wales and Northern Ireland), such as London, Manchester, Edinburgh, Cardiff and so on.
BST is the summer time equivalent of GMT. So for half the year the UK is on Greenwich Standard Time and the rest of the year they observe British Summer Time which is one hour ahead.
While GMT is observed in many places in addition to the UK BST is only observed in Britain (as the name suggests). Not even Ireland are on BST in the summer, instead they have their own time zone for this.