Key difference: In the United Kingdom, GMT is the official time during winter, while BST is the official time during summer. During British Summer Time (BST), the clocks are moved forward during summer and back in the winter.

Time Zones are regions on earth, divided along the longitude, that use the same standard time. The 40 different zones are offset from Coordinated Universal Time (UTC), synonymous with Greenwich Mean Time (GMT), but GMT is no longer precisely defined by the scientific community.

The UTC was standardized in 1961 by the International Radio Consultative Committee and is based on International Atomic Time (TAI); a time standard calculated using a weighted average of signals from atomic clocks located in nearly 70 national laboratories around the world.

GMT is the local time at the 0 degree prime meridian, which passes through Greenwich in London. The Greenwich Mean Time has been the standard time for Britain since 1880. In 1884, GMT was adopted as the universal meridian and hence from it all the standard times of different areas of the globe were calculated. GMT is synonymous with UTC, which is the new universal meridian. However, in the United Kingdom and countries of the Commonwealth, including Australia, South Africa, Nigeria, India, Malaysia, etc., GMT is still the commonly used term.

A number of countries in the northern hemisphere and the southern hemisphere conduct the practice of daylight saving time. Daylight saving time (DST) is the is the practice of advancing clocks so that evenings have more daylight and mornings have less. In daylight savings, the clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn. Daylight saving time is also called summer time in several countries, such as in Britain, and other European countries. British Summer Time (BST) is one such example of DST.

During British Summer Time (BST), the clocks are moved forward during summer and back in the winter. This is done so that the citizens may get more hours of daylight during the day, and hence use less artificial methods of lighting during the evening. BST is the one hour forward advancement of Greenwich Mean Time, essentially, GMT +1.

BST was set up in 1916, when an act was passed ordaining that ‘during a defined period of that year the legal time for general purposes in Great Britain should be one hour in advance of Greenwich Mean Time.’ However, this practice is now under criticism, with opinions divided on the matter of benefits vs. harm of BST.

Basically, in the United Kingdom, GMT is the official time only during winter, while BST is the official time during summer. GMT is the same time as Western European Time. BST usually begins at 1:00 AM GMT on the last Sunday of March and ends at 1:00 AM GMT on the last Sunday of October. In 2012, BST began on 25 March and ended on 28 October. In 2013, it will begin on 31 March and end on 27 October.

Image Courtesy: freelights.co.uk, weavervalleycc.org.uk