If you trace your family roots back far enough, you’ll eventually run across the “double-dating” system, e.g., January 21, 1718/19. The need for this method of dating was the switch from the Julian (old style) to the Gregorian (new style) calendar.

The switch from the Julian calendar to the Gregorin calendar took place October 5, 1582. Thus, October 5, 1582 became October 15, 1582, deleting ten days. This was done to bring uniformity between the calendar and actual time. The vernal equinox was restored by deleting the ten days.The new Gregorian calendar also started the new year on January 1 instead of March 25.

The calendar change from one system to another has caused problems for the American genealogist. Not all countries made the switch at the same time, and some countries were never on the Julian system to begin with. The change in Britain and her colonies, which included most colonies in America, did not take place until 1752. An important year to remember.

“Double-dating” solves the problem when a date falls inclusively between January 1 and March 24, before 1752. An example of this would be two children born to the same mother: one born March 23, 1700; and one born April 5, 1700. Without being aware of the calendar change, you would wonder how a woman could give birth to two children roughly two weeks apart. The actual date of birth for the child born in March is March 23, 1700/01. The difference between April 5, 1700 and March 23, 1700/01 is almost one year.Below is a table of old style dates in chronological order, and the equivalent new style date. Hopefully, this will help clarify why “double-dating” is used.

Old Style	New Style
November 14, 1718	November 14, 1718
December 26, 1718	December 26, 1718
January 3, 1718	January 3, 1718/19
January 22, 1718	January 22, 1718/19
February 16, 1718	February 16, 1718/19
March 5, 1718	March 5, 1718/19
March 23, 1718	March 23, 1718/19
March 28, 1719	March 28, 1719
April 12, 1719	April 12, 1719

You can see looking at the old style dates the new year starts in March, and looking at the new style dates the new year starts in January. If “double-dating” wasn’t used the new style dates from January 1 through March 24 would be off by one year.

In the refernce section below, there are links to web sites that give a more detailed treatment of the subject. “The Perpetual Calendar” contains tables of when various countries switched over to the Gregorian calendar.

Bibliography:

• Val D. Greenwood The Researcher’s Guide to American Genealogy, 2nd Edition, Genealogical Publishing Co., Inc, Baltimore, 1997
• Susan E. Roser Mayflower Increasings, 2nd Edition Genealogical Publishing Co., Inc., Baltimore, 1996

Reference:

• Calendars
• The Perpetual Calendar