The United Kingdom is nearly always 5 hours ahead of US Eastern Standard Time. But in a year when US Daylight Saving Time and BST (British Summer Time) are not synchronized, there will be a week in spring and/or fall when the difference is 4 or 6 hours.

GMT, or Greenwich Mean Time, is the local time of the 0 degree meridian that passes through Greenwich in London, from which the standard times of different areas of the globe are calculated. Thus it is the standard time for Britain, and a basis for other time zones in the world.

Summer time or BST (British Summer Time) runs from the end of March to the end of October (the last Sunday in each month), when clocks are advanced one hour ahead of GMT to gain maximum use of daylight hours.

Additional information on Greenwich Mean Time is available on the Greenwich internet site.