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, as 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, while March 25th was the start of the year, or fiscal year, for many hundreds of years. Both of these conventions changed not very long ago, in historic terms, as just a few centuries ago. So when a date is given in a history book (or an old book), we need to know whether it is in the modern "New Style" or the traditional "Old Style". 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. 