What day was that ancestor born? It seems like such a simple question, and yet finding the answer can be surprisingly complex, even when you have the numbers in front of you. Exact dates are often found in death certificates and frequently on tombstones. The problem is that these are often written as death dates followed by the person’s age at death.
Here is a common example:
Here lies the body of John Smith,
Died August 3, 1904,
Aged 79 years, 9 months, 29 days
How do you tell John Smith’s date of birth?
You obviously need to subtract 79 years and 9 months and 29 days from the date of death. Simple, right? Well, not as simple as it first appears.
First, was it a 31-day month or a 30-day month? Or was it 28 days or 29 days in the case of February?
Next, you have to calculate in Leap Years. Everyone knows that Leap Years occur every four years; but did you know that there is an exception every 100 years? And, just to complicate matters more, there is an exception to that exception every 400 years? Then there is the issue of which calendar was in use. Was it the Julian calendar or the Gregorian calendar?
There are easy solutions to the question.
The simplest answer is to let a computer make the calculation for you. However, if you are in the cemetery or in the courthouse without your computer and an Internet connection, you can use a simple method of determining birth dates by using a handheld calculator. And… if you don’t have a calculator but do remember your grammar school math, you can even make the same calculation with paper and pencil. I’ll describe methods you can use for these calculations wherever you are.
For now, let’s assume that the calendar used is not an issue. If you are calculating 19th, 20th, and 21st century dates in North America, the calendar in use is Gregorian and is not a factor. I will describe the Julian and Gregorian calendars and the conversion dates and locations in a future article.
All of the better genealogy programs of today have the capability to do these calculations for you. If you have a genealogy program installed on your computer and it is readily available, use that.
If you do not have a genealogy program or your program is limited with no date calculator, you can download a small Windows program that will perform the same calculations. The RJT Date Calculator is such a program, described as an aid for family historians. It is a free program although the author does suggest you pay a small donation if you find it useful. You can download the RJT Date Calculator at http://www.taubman.org.uk/datecalc/.
NOTE: The RJT Date Calculator obviously has not been updated in a long time. It is listed as working on “Windows 95/98/NT/2000/ME/XP.” I haven’t tried Windows 8 or 10.
Many of us will find it easier to go to the Internet to find a calculator. The advantages of online apps is that there is no software to install in your computer plus they work on Windows, Macintosh, Linux, Android, Apple iOS, and other operating systems. There are many online birth date calendars available; here are a few that I found:
Birth Date Calculator: http://www.ovs-genealogy.com/tools/free_birthdate_calculator_calculates_birth_day.shtml
Birth Date calculator: http://www.angelfire.com/va/ValsGenealogyPage/Calculator.html
Birth Date Calculator: http://www.longislandgenealogy.com/birth.html
Age Calculator: http://www-users.med.cornell.edu/~spon/picu/calc/agecalc.htm
Birth Date Calculator: http://parallax-viewpoint.blogspot.com/2015/05/measurement-tools.html#BirthDateCalc
Tombstone Birthdate Calculator with Tips: http://www.searchforancestors.com/utility/birthday.html
This is great when you’re sitting in front of a computer; but what do you do if you are in the cemetery with no computer available? You pull out your handheld calculator or paper and pencil and use “Formula 8870.”
NOTE: You do carry a calculator with you at all times, right? I do since a calculator is included as part of my cell phone. Most modern cell phones include built-in calculators. If you have your cell phone in your pocket or purse, you probably have an available calculator, too.
The following is a nifty math trick. You only have to remember a four-digit number and a bit of process.
Using a calculator or a piece of paper, enter the death date as one long series of numbers, starting with the year followed by month and then the day of the month. In other words, write it as yyyymmdd.
For instance, if a person died August 3, 1904, enter the date as: 19040803 (make sure you enter all days and months as 2-digit numbers. Use “03” instead of “3”.
Next, subtract the age at death with the number written in the same manner, using two digits for the years (yymmdd). For a person aged 79 years, 9 months, 29 days, you would write that as: -790929
Now subtract still another number, the constant of 8870
The result is the date of birth, written as: yyyymmdd.
Here is how it looks on paper, using the earlier example of John Smith:
Died August 3, 1904,
Aged 79 years, 9 months, 29 days
19040803 Year, month,and day of death (yyyymmdd)
-790929 Subtract age at death (yymmdd)
-8870 Subtract the constant of 8870
18241004 John Smith was born October 4, 1824 (yyyymmdd)
The drawback is that “Formula 8870” always assumes that every month is 30-days long. The results may be off by as much as two or three days. When recording dates, always refer to them as “about” or “circa.” In the above example, I would record the results as “John Smith was born on or about October 4, 1824.”
You can perform these calculations on paper, on a calculator, or on your cell phone’s built-in calculator. I also keep a text note on my cell phone with the above formula as a reminder when I need it.
Of course, the above programs, web sites, and the “Formula 8870” all assume that the person who recorded the information on the tombstone or on the death record knew the above information and used it accurately. Sadly, you cannot count on that. Clerks and tombstone carvers of a century or more ago did not have access to computers or calculators and may not have known about “Formula 8870.” Even if they did know the formula, mistakes were still common.
I know that the date shown on one of my great-great-grandfather’s tombstone is off by one year from all the census records. I assume that the census takers usually talked directly with great-great-granddad and that the information he provided every ten years probably was accurate. I do know his information was consistent in every census record so I am guessing it was correct. Grieving relatives probably provided the information on his tombstone after his death, and they may have erred in the calculation. There is also a possibility that the tombstone engraver himself made the error. It is theoretically possible that the tombstone is correct and that all the census records were wrong. That seems unlikely, however.
I’d suggest that you first look for a birth record on the exact date calculated by the program/web site/formula. If not found, start looking at records for several days before and after the calculated date. You might also look for the same dates but in both the year before and the year after the calculated date, such as the example of my great-great-grandfather’s tombstone.
In short, use software tools when available. If software is not available at your fingertips at the time of need, use “Formula 8870.” Even then, you must consider the result to be approximate because those who recorded the information years ago may have erred.
May the records you encounter always be accurate.