Until the revolution the old Russian (Julian) calendar was different to that used in the West (the Gregorian calendar). This produced a discrepancy of 12 days in the 19th century and 13 days in the 20th century. The Bolsheviks modernised the calendar, along with much else. The dating of events that occurred before 1918 therefore presents some difficulty. The choice is quite arbitrary and different writers use one system or the other. Whatever other anomalies may be incurred, it seemed undesirable to start referring to the November revolution, the March revolution, and the massacre of 21 January! In general, therefore, I have preferred to keep to the Old Style, but occasionally both dates may be given. To change into the modern calendar, add 13 days (New Style). OS=Old Style; NS=New Style.