The Julian calendar (Old Style), introduced by Julius Caesar in 46 B.C. and adopted by the First Council of Nicaea in A.D. 325, was used in Russia up to the Revolution of 1917. The Gregorian calendar (New Style), now in general use, was introduced by Pope Gregory XIII in 1582. The date October 5, 1582, was called October 15, 1582; thus ten days were dropped. In Great Britain, however, the Old Style lasted till 1752, when, in September, eleven days were dropped.
The years 1700 and 1800 were not leap years by the Gregorian rules (whereas 1600, being divisible by 400, was); therefore, the difference between the two calendars was increased in each of those years by one day, bringing it to eleven days from 1700 to 1800, twelve from 1800 to 1900, and thirteen from 1900 to 1917. Thus the middle of July in Russia would be the end of July elsewhere, while January 12, 1799, and January 13, 1800, in the world at large would both be New Year’s Day in Russia.
In the present work all dates pertaining to events in Russia are Old Style unless stated otherwise. Dates pertaining to events in the rest of the world are New Style. When there exists a possibility of confusion, both styles are given thus: 1/13 January.