This maps to 11 July (Gregorian calendar), conveniently close to the Julian date of the subsequent [and more decisive] Battle of Aughrim on 12 July 1691.This latter battle was commemorated annually throughout the eighteenth century on 12 July, following the usual historical convention of commemorating events of that period within Great Britain and Ireland by mapping the Julian date directly onto the modern Gregorian calendar date (as happens for example with Guy Fawkes Night on 5 November).Because of the differences, English people and their correspondents often employed two dates, dual dating, more or less automatically.For this reason, letters concerning diplomacy and international trade sometimes bore both Julian and Gregorian dates to prevent confusion: for example, Sir William Boswell writing to Sir John Coke from The Hague dated a letter "12/22 Dec. In his biography of Dr John Dee, The Queen's Conjurer, Benjamin Woolley surmises that because Dee fought unsuccessfully for England to embrace the 1583/84 date set for the change, "England remained outside the Gregorian system for a further 170 years, communications during that period customarily carrying two dates".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", to indicate that in the following few weeks the year was 1661 Old Style but 1662 New Style.
For example, the Battle of Agincourt is universally known to have been fought on 25 October 1415, which is Saint Crispin's Day.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. 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.The need for change arose from the realisation that the correct figure for number of days in a year is not 365.25 (365 days 6 hours) as supposed by the Julian calendar but almost exactly 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year: the Julian calendar has too many leap years. When this usage is encountered, the reader should not assume that the British adoption date is intended, or that the 'start of year' change and the calendar system change were adopted concurrently, or even that religious adoption accompanied civil adoption.But for the period between the first introduction of the Gregorian calendar on 15 October 1582 and its introduction in Britain on 14 September 1752, there can be considerable confusion between events in continental western Europe and in British domains.Events in continental western Europe are usually reported in English language histories as happening under the Gregorian calendar.