Dual Dating and the Gregorian Calendar Conversion in Great Britain and its Colonies


© 2011 Troy L. Adamson



            Dual dating is a confusing concept misunderstood by many. The Internet contains much information about the topic, but few sources present the information properly. In fact, there is a lot of misinformation on the Web that serves only to confuse the subject even further. To clear up any confusion regarding the dating conventions used, I have attempted to explain, in simple terms, the concept of dual dating as it applies to historical documentation.

Dual dating was a means of documenting dates using both the Julian and Gregorian calendars. In 45 BC, Julius Caesar brought about the Julian calendar as a reform of the Roman calendar. Pope Gregory XIII introduced the Gregorian calendar on 24 February 1582, but the first countries to use the new calendar did not adopt it until 15 October 1582. Over the next several years, more and more countries converted to the Gregorian calendar with Turkey being the last in 1926. After continued resistance against adopting a Catholic invention, Britain finally converted to the Gregorian calendar on 14 September 1752, so starting with this date, dual dating no longer applied to Britain and its colonies.

From the period of 15 October 1582 (the official start of the Gregorian calendar) to 13 September 1752 (the day before the start of Britain’s use of the calendar), Britain and its colonies used a system of dual dating to represent the dates of both calendars. It is worth noting that historians do not generally use the Gregorian calendar when recording dates prior to its adoption on 15 October 1582. Therefore, dual dating does not apply before 15 October 1582.


Dual Day Dating


When dual dating, two adjustments occur: the day of the month and the year. When the Gregorian calendar was created in 1582, it was realized that the Julian calendar was 10 days out of synch with the solar year. To restore proper synchronization with the seasons, the new calendar dropped 10 days from the month of October in 1582, and to prevent the problem of extra days from occurring again, 1 day was added to February for every year divisible by four (leap year). Leap years add a 29th day to February, which normally has 28 days. Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by one hundred. The Gregorian calendar also omits 3 leap days every 400 years. The leap year correction meant that up until Britain adopted the Gregorian calendar on 14 September 1752, additional days had to be added to each recorded date. The proper number of added days depended on the year. The following diagram illustrates the number of days to be added when converting from Julian to Gregorian calendar dates:


Julian Range

Gregorian Range


From 5 October 1582 to 28 February 1700

From 15 October 1582 to 10 March 1700

10 days

From 29 February 1700 to 28 February 1800

From 11 March 1700 to 11 March 1800

11 days

From 29 February 1800 to 28 February 1900

From 12 March 1800 to 12 March 1900

12 days

From 29 February 1900 to 28 February 2100

From 13 March 1900 to 13 March 2100

13 days

From 29 February 2100 to 28 February 2200

From 14 March 2100 to 14 March 2200

14 days


When converting from the Julian to the Gregorian calendar, the date, 11 June 1690, would be 21 June 1690. 10 days are added because there is a difference of 10 days for the year 1690. To properly record this date using dual day dating, a slash is placed between the Julian and Gregorian calendar days; 11/21 June 1690.         


Dual Year Dating


The period between 1 January and 24 March, and between the years 1583 and 1752, was subject to dual year dating because the Gregorian calendar changed the New Year from 25 March to 1 January. To accommodate this change, dates had to reflect years in both Julian and Gregorian terms. If a date fell between 1 January and 24 March, and between the years 1583 and 1752, it had to be represented with dual years. To represent a dual year, one year was added to the Julian year. For example, the date, 15 February 1701 would have been written, 15/26 February 1701/02. The years for dates that ranged from 25 March through 31 December, and between the years 1583 and 1752, remained the same.   

An excellent automated calendar converter can be found at the following website: http://calendarhome.com/converter/


Old Style and New Style Dates


            Dates can also be represented by adding the notation, “Old Style” (O.S.) and “New Style” (N.S.), to historical dates for clarification of the calendar system used; Old Style, referencing the Julian calendar and New Style, the Gregorian calendar. So, a distinction between the two dating systems would be written: 20 January 1718 O.S. and 31 January 1719 N.S.


Sample Date Conversions


Old Style Date

New Style Date

Dual Date

10 January 1690 O.S.

20 January 1691 N.S.

10/20 January 1690/91

12 February 1701 O.S.

23 February 1702 N.S.

12/23 February 1701/02

15 March 1715 O.S.

26 March 1716 N.S.

15/26 March 1715/16

18 April 1723 O.S.

29 April 1723 N.S.

18/29 April 1723


Dual Dating in Scotland


Scotland became part of the Kingdom of Great Britain on 1 May 1707, so like the rest of Britain, the Gregorian calendar was not used in Scotland until 14 September 1752. However, Scotland changed its New Year to 1 January in 1600, meaning dual “year” dating was not needed for Scottish dates beginning 1 January 1600. Like Britain, dual “day” dating still applied until 14 September 1752. The same date conversion rules applied to Ireland as they did Britain.      


English Regnal Years


Some English documents were dated using regnal years; the period by which a ruling monarch served on the throne. In England and its colonies, regnal years were occasionally used as time markers. 1 Henry VII implies the first year reign of Henry VII. Henry VII came to the throne on 22 August 1485, so 1 Henry VII implies the period of time between 22 August 1485 and 21 August 1486.

English and early American documents sometimes include wording such as, “the fifteenth of May, in ye fourteenth year of His Majesty’s reign, George.” The fourteenth year references year fourteen of King George II’s reign. Since George II came to the throne 11 June 1727, the fourteenth year of his reign would be 1741. The same rules applied for English regnal years; dates that fell between 5 October 1582 O.S. and 2 September 1752 O.S. were subject to dual dating.   

An easy-to-use regnal year to calendar year automated converter can be found at the following website: http://www.genproxy.co.uk/king_queen_reign_dates_regnal.htm  


The Quaker Calendar


            Prior to 1752, the Religious Society of Friends, more commonly known as Quakers, subscribed to the Julian calendar like the rest of their British counterparts with one exception; they used numbers to denominate the names of the months and days of the week. Sunday became the first day, Monday, the second day, etc. After 1752, January became the first month, February, the second month, etc. This calendar numbering system was known as the “plain calendar;” sometimes called the “scriptural calendar.” The plain calendar was an alternative to the “world’s calendar,” which used traditional names derived from pagan deities. Though the plain calendar is associated with Quakers, it was actually not developed by them, but rather from the general nonconformist movement that swept through England during the 17th century.   

            Quakers typically wrote dates as, “12th da 5th mo 1722.” The month was occasionally written using Roman numerals. Since prior to 14 September 1752 the British New Year was 25 March, March was considered the first month of the year. April was the second month, May, the third month, and so forth. See the naming pattern in the table below.


Quaker Month Conversion for Dates Prior to 14 Sept 1752


Quaker Month

Converted Month

1st month


2nd month


3rd month


4th month


5th month


6th month


7th month


8th month


9th month


10th month


11th month


12th month



So, a Quaker born the 12th day, 12th month, 1656, would have been born 12/22 February, 1656/57. Notation for Quaker dates often uses the following formats: 12/22 xii [February] 1656/57, or 12/22 12 mo. [February] 1656/57. When converting Quaker dates, convert the month first, then convert to the Gregorian calendar.


Sample Quaker Date Conversions


Quaker Date

Gregorian Conversion Date

10th day, 7th month, 1677

10/20 7 mo. [September] 1677

21st day, 1st month, 1702

21/1 1 mo. [April] 1702

5th day, 11th month, 1718

5/16 11 mo. [January] 1718/19

7th day, 1st month, 1733

7/18 1 mo. [March] 1733/34





 History of page:

Change on Scotland paragraph 5/19/2010

Complete revision    3/7/2011