Byzantine calendar
Peter the great became tsar of Russia in 1682. At the time the Russians used a form of the Julian calendar with Byzantine influence. The calendar year started on 1 September and not 1 January. The years were numbered from the date given as earth's creation and not from the birth of Christ. Peter changed the calendar to start on January 1 and numbered the years from Christ's birth. But he continued basing the calendar on the Julian system instead of the Gregorian calendar. By the early 1800s the Russian calendar was 12 days behind the Gregorian calendar used by western Europe. So dating something between the two systems became a problem. Dating notations such as 1 March O.S. (meaning Old Style, Julian or Russian) were used. Another example is 16 May 1582 N.S. (New Style or Gregorian calendar). Other notations included both: 25 October/6 November 1917, trying to show how the same day appeared in different calendars.