Julian to Gregorian Calendar change and the affect on dates

Because the use of double-dating of calendar dates between 1582-1752 might be misinterpreted — i.e., dates expressed as dd Mmm yyyy/yy (e.g., 20 Mar 1634/35) thought of as an expression of uncertainty as to the year — the following is a brief explanation:

The calendar that Julius Caesar devised for the Romans in 46 B.C. was based on a year of 365.25 days (extra day added every four years) whereas the actual astronomical time was 365.2422 days. This difference only amounted to 11 minutes 14 seconds per year but in 125 years that equaled a whole day. By 1582 the Julian Calendar had accrued an error of 10 days which threw off the date of Easter. Pope Gregory III, head of the Roman Catholic Church, ordered the ten days dropped, the leap year day omitted every 400 years and changed New Year’s Day from 25th of March to the 1st of January.

But in 1582 England, not being a Catholic country, did not accept the new Gregorian calendar. During the next 170 years England, and later America, used two calendars — the ecclesiastical Julian calendar with its New Year’s Day on 25 March (O.S. = old style) and the new Gregorian calendar with its new year beginning on 1 January (N.S. = new style).

In 1752 when the error in the Julian calendar amounted to eleven days, England converted to the Gregorian calendar, as had the rest of Europe, by eliminating the eleven days between the 2nd and 14th of September (e.g. the date 7 September 1752 does not exist). With the change to the new calendar, New Year’s day moved from 25 March to 1 January. This meant that the months of January, February & March (through the 25th) in the years 1582-1752 have a double year.

Thus a date expressed as 20 Mar 1634/35 is not ambiguous — the year was 1634 on the old Julian calendar and 1635 on the new Gregorian calendar. A date of May 1635 has only one year because it is after the Julian New Year’s Day of 25 March.

However, an original date expressed as 20 Mar 1634 is ambiguous. This may arise when the custom of a recording body was either O.S or N.S. — understood at the time but not marked in the document (and perhaps not known today). This date could either be 20 Mar 1634/35 (if the original document was dated O.S.) or 20 Mar 1633/34 (if the original document was dated N.S.), a difference of a year. Another instance where an original date is ambiguous is when it is expressed as “on the 20th, 12th mo., 1634.” The 12th month is March if O.S., and December if N.S.