This article discusses how to convert numeric months into the appropriate month name with old style dates.
Changing When the Year Starts
Europe inherited from Roman times a legal year that started on 25 March. Wikipedia has information on how various countries changed over to a legal year starting on 1 January. This article focuses on Britain and British colonies, where prior to 1752, years started on 25 March, with March considered the first month. By law, 1751 was a short year, and 1752 started 1 Jan, with January now the first month. Though the date of changeover may be different in other countries, the principles should be the same.
In years prior to 1752, the year started with March, then April, May, ... December, January and ended with February as shown in the table below. Therefore, when you see month number 1, it was not January as we are used to, but March. Notice that this causes the months with numerically-based names to agree with their number under the old system: September is the 7th month, October the 8th, November the 9th, and December the 10th. The most difficult part of this translation is at the tail end, where month 11 was January and part of the previous year, compared to our modern calendar, and month 12 was February, the last month of the previous year.
Table of Month Numbers
Interpreting Numeric Months
Just as today, many writers of the time would choose to represent dates with numbers. Anybody that is working in genealogy with dates before 1752 must understand the shift in the beginning of the year so they can properly convert such numeric month designations to the correct month name.
Unless a date is marked by an indication that it is new style (n.s. or N.S., usually), the interpretation of month numbers before 1752 should follow the Old Style column in the above table. That was the standard of the time.
Here are some example dates as they might be recorded prior to 1752, with the appropriate interpretation.
Notes on Double Dating
Writing old style dates in the period from 1 Jan to 24 Mar can be ambiguous. For a date like 18 Feb 1662, readers will wonder if 1662 refers to the year as used then, or if it has been adjusted to the modern year. (Many writers like to use the modern year numbering, for various reasons. For example, it allows simple comparison and age calculations. See here for one such justification.)
To communicate the exact date precisely without ambiguity, double-dating is used. Thus, the above date becomes either 18 Feb 1661/62 or 18 Feb 1662/63 depending on which of the two interpretations is used. 18 Feb 1661/62 says the date is 18 Feb in the year that was then 1661 but is 1662 in the modern system, while 18 Feb 1662/63 is one year later.
Double dating is not needed for dates 25 Mar or after, since both the old and new styles agree that it belongs to the same year. Sometimes, inappropriate use of double-dating may be a sign of an error. For example 24 Jun 1662/63 could indicate that the month was really January.
Deciding which interpretation was meant can be difficult, but some generalizations are possible.
Notes on Source Citations
In date fields, WeRelate guidelines suggest dates be put in the form DD MMM YYYY with 3-letter month abbreviation. Using all numeric dates is explicitly discouraged. For numeric dates, the user is expected to make the appropriate translation to the correct month name.
When a source gives a numeric date, the source citation should preserve that format so that future readers may see both the original as well as the interpreted form.
For example, the text of a source citation of a genealogy book might say the following: