Many British people continued to celebrate their holidays "Old Style" well into the 19th century, a practice that according to the author Karen Bellenir reveals a deep emotional resistance to calendar reform. It is common in English language publications to use the familiar Old Style and/or New Style terms when discussing events and personalities in other countries, especially with reference to the Russian Empire and the very-early Russian Soviet.
The need for change arose from the realisation that the correct figure for the number of days in a year is not 365.25 (365 days 6 hours) as supposed by the Julian calendar but rather less: the Julian calendar has too many leap years. For example, in the article "The October (November) Revolution" the Encyclopædia Britannica uses the format of "25 October (7 November, New Style)" to describe the date of the start of the revolution.
Friday, 6 October 1867 was followed by Friday, 18 October.
Instead of 12 days, only 11 were skipped, and the day of the week was repeated on successive days, because at the same time the International Date Line was moved, from following Alaska's eastern border with Canada to following its new western border, now with Russia.
In contrast, Thomas Jefferson, who lived during the time that the British Isles and colonies eventually converted to the Gregorian calendar, instructed that his tombstone bear his date of birth using the Julian calendar (notated O. for Old Style) and his date of death using the Gregorian calendar.
From 1155 to 1752, the civil or legal year in England began on 25 March (Lady Day) The corresponding date in the Gregorian calendar is 9 February 1649, the date by which his contemporaries in some parts of continental Europe would have recorded his execution. During the years between the first introduction of the Gregorian calendar in continental Europe and its introduction in Britain, contemporary usage in England started to change.
Closely related is the custom of dual dating, where writers gave two consecutive years to reflect differences in the starting date of the year, or to include both the Julian and Gregorian dates.
Beginning in 1582, the Gregorian calendar replaced the Julian in Roman Catholic countries.
To reduce misunderstandings about the date, it was normal in parish registers to place a new year heading after 24 March (for example "1661") and another heading at the end of the following December, "1661/62", a form of dual dating to indicate that in the following few weeks the year was 1661 Old Style but 1662 New Style.
Some more modern sources, often more academic ones, also use the "1661/62" style for the period between 1 January and 25 March for years before the introduction of the Gregorian calendar in England. and the British Calendar (New Style) Act 1750, Ireland, Great Britain and the British Empire (including much of what is now the eastern part of the United States) adopted the Gregorian calendar in 1752, by which time it was necessary to correct by 11 days.
Wednesday, 2 September 1752, was followed by Thursday, 14 September 1752.