Old Style (or O.S.) and New Style (or N.S.) are terms used for calendar dates in English language historical studies, for two reasons. The first reason is that the method of dating that is most widely used around the world today, the Gregorian calendar, was introduced into English cultures only in 1752. The second is that 1 January has not always been the first day of the year: in the British Empire (and some other countries), March 25th was the start of the year. Both of these conventions changed just a few centuries ago, when the Calendar (New Style) Act 1750 was made law. The dating style used before 1752Julian Calendar, year beginning 25 March is called Old Style Dating and the dating style used todayGregorian Calendar, year beginning 1 January is called New Style Dating. So when a date is given in a history book (or an old book), we need to know whether it is using 'New Style' or 'Old Style' dating. During the time of the changeover, people would give both dates. Even today, when historians are writing about an event in those times, they often give the date as it was used at the time but also give the modern equivalent for your convenience. 