Recording dates to reflect the calendar change

If you start doing genealogical research back into the 1700s you will likely run into an interesting problem concerning the change from the Julian Calendar to the Gregorian Calendar that began in 1582 A. D and continued until as late as 1752 depending on the country. This so-called calendar change or adjustment, presents an additional interesting problem in the way to record dates both before and after the change. When genealogists are dealing with places, the rule is to record the name of the place and its jurisdiction as it was at the time the event occurred. However, there is a question as to whether the same type of rule applies to dates. If the event occurred in the transition period, do you record the date according to the Julian Calendar or the Gregorian Calendar or both? The answer to this question turns out to be quite complex and a real challenge to genealogists unaware of the issue.

The Julian Calendar was used throughout Europe beginning as early as 45 B.C. In that calendar system, the new year began on March 25 and March was the "first month" of the new year. The problem arose from the fact that the Julian Calendar to not take into account the difference between the length of the year and actual length of the year according to the rotation of the earth around the sun. As a result, the seasonal dates, such as the calculation for Easter, slowly moved later in the year.

Beginning in 1582, under the mandate of Pope Gregory XIII, the calendar was revised and correct for the drift, the last day of the Julian Calendar was 4 October and the following day was 15 October. Unfortunately, this change was not accepted by non-Catholic countries for up to 170 years. As time passed, the number of days needed to correct the Julian or Old Style Calendar increased.

Illustrated below in this Table from the Wikipedia article "Old Style and New Style dates," are the number of days needed to correct the Old Style dates to New Style dates. Researchers also have to take into account that the days between the adjustment dates simply disappear from the calendar.

Conversion from Julian to Gregorian dates

Time period (from 1 March of first year to 28 February of last year)	Сorrection, days
1–100	−2
100–200	−1
200–300	0
300–500	+1
500–600	+2
600–700	+3
700–900	+4
900–1000	+5
1000–1100	+6
1100–1300	+7
1300–1400	+8
1400–1500	+9
1500–1700	+10
1700–1800	+11
1800–1900	+12
1900–2100	+13
2100–2200	+14

If all this sounds complicated, it is. But ignoring this issue causes some really interesting dating problems for genealogists

Now the issue becomes, how do genealogists record their date entries to match the changes made to the calendar? Here is a partial explanation from the Encyclopedia of Genealogy:

Around 1700 many New England town clerks began entering split-year dates. If one is found to be clearly entered, then use "21 February 1704/5" or whatever form the recorder used. For dates in the ambiguous, 1 January-24 March range without split dates, safe genealogical practice suggests using the notation "21 February 1704[/5]" when primary, direct evidence clearly suggests a split year applies, with an explanation in your footnote. If there is primary, indirect evidence that the split date applies but was omitted, then "21 February 1704[/5?]" would be appropriate, again with an explanation in a footnote. In general, the acceptable practice is to not convert pre-Gregorian dates to New Style, or (N. S.), but rather to record Julian dates with an Old Style, or (O. S.) notation suffix, for example, "She dated her letter 21 February 1714 (O.S.)". Conversion simply introduces another error opportunity, and further removes the information from its original form. Other genealogists can handle the conversion, if they have the need.

Researchers in America need to understand this because the English Colonies, including those in America, did not make the change until 1752 but implementation depended on the exact area, some beginning before 1752 and some after.

Unfortunately, there are few of the genealogy programs that make allowances for the split-year dates and the best you can expect is that the program will let you enter a "non-standard" date to accurately reflect the issue.

For more information on the subject see the following sources:

• Encyclopedia of Genealogy: Calendar Change - 16th Century
• Connecticut State Library: The 1752 Calendar Change
• Ancestor Search: Perpetual Calendar Calculator
• Wikipedia: Old Style and New Style dates
• Spathanky, Mike, Old Style and New Style Dates and the change to the Gregorian Calendar: A summary for genealogists 

There are dozens, perhaps hundreds of more articles on the same subject; try searching for "calendar change genealogy" in Google.

For a more extensive discussion, including tables for calculating dates, see:

Cheney, C. R., and Michael Jones. A Handbook of Dates: For Students of British History. Cambridge, U.K.: Cambridge University Press, 2000.

This whole discussion, of course, does not address the issue of other calendar systems throughout the world, including the Jewish Calendar or other non-European systems.