Great Britain, along with its overseas possessions, adopts the Gregorian calendar.

Great Britain, also known as the Kingdom of Great Britain, was a sovereign state in Western Europe from 1707 to the end of 1800. The state was created by the 1706 Treaty of Union and ratified by the Acts of Union 1707, which united the Kingdom of England and the Kingdom of Scotland to form a single kingdom encompassing the whole island of Great Britain and its outlying islands, with the exception of the Isle of Man and the Channel Islands. The unitary state was governed by a single parliament at the Palace of Westminster, but distinct legal systems—English law and Scots law—remained in use, as did distinct educational systems and religious institutions, namely the Church of England and the Church of Scotland remaining as the national churches of England and Scotland respectively.

The British Empire comprised the dominions, colonies, protectorates, mandates, and other territories ruled or administered by the United Kingdom and its predecessor states. It began with the overseas possessions and trading posts established by England in the late 16th and early 17th centuries, and colonisation attempts by Scotland during the 17th century. At its height in the 19th and early 20th centuries, it became the largest empire in history and, for a century, was the foremost global power. By 1913, the British Empire held sway over 412 million people, 23 percent of the world population at the time, and by 1920, it covered 35.5 million km² (13.7 million sq mi), 24 per cent of the Earth's total land area. As a result, its constitutional, legal, linguistic, and cultural legacy is widespread. At the peak of its power, it was described as "the empire on which the sun never sets", as the sun was always shining on at least one of its territories.

The adoption of the Gregorian Calendar has taken place in the history of most cultures and societies around the world, marking a change from one of various traditional dating systems to the contemporary system – the Gregorian calendar – which is widely used around the world today. Some states adopted the new calendar in 1582, others not before the early twentieth century, and others at various dates between. A few have yet to do so, but except for these, the Gregorian calendar is now the world's universal civil calendar, old style calendars remaining in use in religious or traditional contexts. During – and for some time after – the transition between systems, it has been common to use the terms "Old Style" and "New Style" when giving dates, to indicate which calendar was used to reckon them.