As Glenn Test observes in his narrative on John Test, the records of the Quarterly Meeting of Friends in London gives us the birth date of Elizabeth Test (daughter of Thomas and Elizabeth Test) as 12mo, 12day, 1656. There are three problems involved with understanding how this date compares to our modern calendar.
I. The first problem stems from the Quaker tradition of avoiding the use of the pagan names for the months. To avoid pagan names, they numbered the months, 1st month, 2nd month, ... 12th month. This leads us to the problem.
Until 1752, England and its colonies dated events according to the Julian Calendar. According to the Julian Calendar, March was the 1st month of the year and February was the 12th month of the year. So in general, for any year prior to 1752 when the modern Gregorian Calendar was adopted, "12th month, 12th day", would be translated as "February 12" rather than as "December 12".
The tradition of calling March the 1st month is consistent with the meanings of the (Latin) sept-seven, octa-eight, nonus-ninth, and (Greek) deca-ten. Thus, the Julian Calendar September is the 7th month, October is the 8th month, November is the 9th month, and December is the 10th month. Continuing the sequence, January is the 11th month and February is the 12th month.
Another aspect of this problem is that in England and its colonies, March 25 was considered the first day of the year. So the date 12mo, 12day, 1656 should be understood as February 12, 1657. That is, by the Julian Calendar the year was still 1656 but by the modern Gregorian Calendar the year was 1657.
II. The second problem. According to the Julian Calendar the average year is 365.25 days. Every year is 365 days except that every fourth year is a leap year with an extra day making it 366 days. But this meant that the Julian Calendar adds a day about every 130 years in comparison with the modern Gregorian Calendar which includes a leap year every four years except at the turn of each century, which are leap years only if the year is divisible by 400 (e.g., 1600, 2000, and 2400). This makes the average Gregorian year 365.24250 days, i.e., 365 days, 5 hours, 49 minutes and 12 seconds.
The Nicaean Council of 325 C.E. had determined that the vernal equinox should occur on March 21st of every year. But the Julian Calendar rule of adding an extra day to the calendar every four years, resulted in the date of the vernal equinox occurring on March 10, 1752 rather than on March 21, 1752 -- a "slippage" of eleven days.
Most of the Catholic countries adopted the Gregorian Calendar upon its promulgation in the sixteenth century. And by 1701 just about all of the continent of Europe had adopted it. By the time King George II of England, ordered the adoption of the Gregorian Calendar in 1751, it was necessary that eleven days be dropped--jumping from September 2, 1752 to September 14, 1752 (that is skipping September 3, 4, 5...13 for a total of eleven days).
Because of the "loss of days" required to return the equinox to March 21, there are two steps involved in transcribing the Quaker 12mo, 12day, 1656 or any other Julian Calendar date to a modern Gregorian date. The first step is to understand that the Julian calendar 12th month is February. As a corollary, since the modern calendar begins each year on January 1 rather than on March 25, the Julian 12th month, 1656 should be understood as equivalent to the Gregorian date of February 1657.
The second step is to calculate the appropriate day of the month. At the time The Society of Friends or Quakers came into existence and started keeping records, the Julian Calendar was ten days off. Thus 12mo, 12day, 1656 on the Julian Calendar is equal to February 22, 1657 on the Gregorian Calendar. After 1700 (a leap year on the Julian but not on the Gregorian Calendar), eleven days need to be added to the Julian date to determine the Gregorian date. And, of course, February 29 (or 12th month, 29th day, 1700) should be rendered as March 11, 1701.
III. The third problem would not exist if everyone had followed George II's edict and adopted the Gregorian Calendar. Unfortunately, many people continued to use the Julian Calendar after 1752. In some cases the user of the Julian Calendar indicated this fact by including the abbreviation "O.S." for "Old Style" with the date and sometimes those who adopted the Gregorian Calendar included the notation "N.S." to indicate use of the "New Style" or Gregorian dating system.There is some consistency in the old records which may be found wanting in the secondary material. In some cases dates in the original records were entered as, e.g., 12th mo, 12th day, 1753/54 indicating that Julian calendar was still being used and that the year was 1753 in the Julian Calendar but 1754 in the Gregorian Calendar. This method of dual dating for the year would only be found for the months of January (11th month), February (12th month) and March 1-24 (1st month).
With secondary material, especially with genealogies compiled by later generations, the problems can be multiplied. For example, a date in the original records may appear as 12th mo 12th day 1753/54. An interpreter of such a record might have entered this date as December 27, 1753/54 not realizing that the use of the dual year notation implies that the Julian Calendar is being used and that 12th month is February and that the year is our Gregorian 1754.
The simplest solution to these problems is always to quote the original record. If the original record says "1st mo 21st day 1654" then enter the date exactly that way, indicating that this is exactly how the record is written. Only if we want to correlate this date with the modern calendar do we need to enter it any differently. Entering it as March 21, 1654 or even March 21, 1655 does not, however, correlate to our modern method of dating.
A person born on March 21st 1654 (Julian Calendar) was born 10 days after the vernal equinox. If we are to place this birth accurately on the modern calendar we need to date it as March 31, 10 days after the vernal equinox, in the year 1655, since the modern calendar year begins on January 1 rather than March 25.
Since confusion is likely to reign, accurate quoting of the record is important. The practice should be to quote the record. Translating the date properly to modern Gregorian Calendar is a secondary option. In every instance of a date prior to 1652, when the calendar was changed, the date should be specifically entered to indicate that it is either a Gregorian date or a Julian date.
References
Calendar Calculator for converting Julian Dates to Gregorian Dates