Basic Genealogy: Calculating Dates

If our genealogical quests were easy, all pertinent information about an ancestor would be found in one place and would be well documented. Unfortunately, the records left behind by our ancestors were not recorded for our benefit and this requires us to do a little work from time to time.

Commonly called BMD (birth, marriage, and death) records, vital record information may not be complete, available, or even accessible. In some jurisdictions, civil registration was not mandatory until the late 19th or early 20th centuries. Because of this, accurate dates of birth may not always be available.

In the next several posts, I hope to give you some insight on how to use various records that are available to help you at least pinpoint a year of birth if not an actual date. This particular article will deal with the calculating a birth date when an age at the time of death is known. Often these notations are found on tombstones and they will help us to determine a birth date of an ancestor.

There are two methods to do this and I will show you both before I give you the easiest method of calculating a birth date. I am also providing three examples and will work the math for you.

The first method is the standard method where the birth date is determined by subtracting the years, months, and days from the date in three calculations.

The second method, called the 8870 method, calculates the birth date by converting the three sets of numbers into single numbers, subtracting the age at death from the death date, and then subtracting the constant of 8870 to correct for errors.

I’ll work out both methods with three actual examples.

Example One: John S. Myers

My third great grandfather, John S. Myers is buried in Independence–Butler Cemetery in Butler, Richland County, Ohio. I visited this graveyard in July 2007 and was able to photograph his stone as well as other relatives buried in this location. Like John, several of the older stones provided a date of death and the subject’s age at the time of his or her demise. In this case, John S. Myers died on November 26, 1878. His reported age was 74 years, 8 months, and 18 days.

Standard Method

	YEAR	MONTH	DAY
Death Date	1878	11	26
Age at Death	-74	-8	-18
Birth Date	1804	3	8

Using the standard method, John’s birth date is calculated as being March 8, 1804.

8870 Method

This particular method requires you to convert the date of death to an eight digit number and the age at death to six or seven digit number. Seven digit numbers will only be used for those whose age at death is 100 years or greater. Regarding the death date calculation, John’s November 26, 1878 death date is converted in the following method:

First, the year constitutes the first four digits 1874.

Second, the month is digits six and seven: 11 (for months with single digits, i.e., May being 5 – precede the number with a 0. Therefore, May would be 05; August = 08; September is 09.

Third, the final two digits are the day. In this example, it is 26. Like the month, single digit numbers like 2, 4, and 9 must be preceded by a 0 making them 02, 04, and 09.

The calculated number is 18741126

Similarly, the age of death is also constructed in the same manner. If there is no month or day date, 00 must be used. John’s age was 74 years, 8 months, and 18 days. Therefore, the calculated number is 740818.

In the 8870 method, the date of death is subtracted from the age; the subtotal is subtracted by the constant of 8870 and the resultant number is the birth date. Because the constant is used, the numbers may need to be adjusted.

DEATH DATE NUMBER	18781126
AGE NUMBER	-740818
SUBTOTAL	18040308
CONSTANT	-8870
TOTAL	18031438

Note in the above number there are some anomalies in the days and months. If the number of days is over 30, subtract 30 (don’t worry about 28, 29, and 31 day months at this step). The resulting number is 08.

The 30 subtracted from the days is converted to one month which needs to be added to the day columns – making 15 months. Since the number 15 is beyond 1 year, subtract 12 from this number and the result is 03. The 12 months are converted to one year and this year is added to the year number of 1803 making the result 1804.

The adjusted number is 18040308 or March 8, 1804.  While it occasionally occurs as it did here, the subtotal is the correct date of birth.

Example Two: Mary Myers

My third great grandmother, Mary Myers is buried in same cemetery as her husband John S. Myers. Her information appears on the opposite side of the monument marking their graves. Mary died on May 1, 1879 – a little over six months after her husband. Her age at death was 74 years, 11 months, and 14 days.

Because the numbers for the age in days and months is greater than the days and months in the date of death, this must be adjusted to perform the calculation. Since the day attribute of 01 is too small, we need to borrow 1 month or 30 days. Therefore, the months are reduced to 04 and the days increased to 31.

Similarly, the month number of 04 is too small. We need to borrow one year; this will add 12 months making the month 16 and the year becomes 1878.

Standard Method

	YEAR	MONTH	DAY
Death Date	1879	05	01
Adjusted Death Date	1878	16	31
Age at Death	-74	-11	-14
Birth Date	1804	5	17

With the standard method, Mary Myers’ birth date is reckoned as May 17, 1804.

8870 Method

In this example, the 8870 method is fairly straightforward and requires no additional calculation.

DEATH DATE NUMBER	18790501
AGE NUMBER	-741114
SUBTOTAL	18049387
CONSTANT	-8870
TOTAL	18040517

The 8870 method also calculates the age as May 17, 1804.

Example Three: John C. Brakeall

My great-great grandfather, John C. Brakeall, is buried in the Antioch Christian Church Cemetery at Big Cove Tannery, Fulton County, PA. I had an opportunity to visit this cemetery during October 2005. It is a quaint little graveyard that is up on a long ridge. While this is generally called the Antioch Cemetery, it is actually two cemeteries: the Antioch Church Cemetery and the Zion Methodist Church Cemetery. I believe this grave is located on the Methodist side. The Zion Methodist Church no longer exists and hasn't for some time.

His date of death is listed on his tombstone as December 23, 1896 and he was 63 years and 23 days at the time of his death.

Standard Method

	YEAR	MONTH	DAY
Death Date	1896	12	23
Age at Death	-63	-0	-23
Birth Date	1833	12	0

Notice the anomaly in the above date. It has the calculation as December 0, 1833. Since this is obviously impossible, the date should be moved to the last day of the previous month making his birth date as November 30, 1833.

8870 Method

In this example, the age at death needs to be adjusted adding 12 months and subtracting one year. Thus, 630023 becomes 621223. You cannot have 00 in the month position; however, it can appear in the day position.

DEATH DATE NUMBER	18961223
AGE NUMBER	-621223
SUBTOTAL	18340000
CONSTANT	-8870
TOTAL	18331130

John’s birth date calculation arrives at November 30, 1833 by using the 8870 method.

The Quick & Easy Method

Just like most college instructors, I have explained how to calculate birth dates using simple mathematical equations. Fortunately, you do not have to perform the above procedures as there are online calculators that do the work for you.

You are asked to input the date of death and the age at death and once you click the calculation button, the date of birth will be displayed.

One of my favorite tools is found at the ProGenealogists’ web site. You can find it at http://www.progenealogists.com/birthfromdeath.htm.

I hope that you found this helpful.