- Newsletter of the BCG
Amy Larner Giroux, "Date Calculations,"
OnBoard 9 (May 2003): 12-13,10.
Names and dates, names and dates… These
are the building blocks of family trees. Often, beginning
genealogists only consider names, collecting ancestors—or
those they think are ancestors—without taking the
time to collect multiple sources of information and verifying
dates. Sometimes individuals with the same name and approximate
birth date are tacked onto the wrong tree.
Analyzing event dates in order to determine birth dates
or ranges is a crucial part of genealogy. The arithmetic
behind these calculations is fairly simple. Problems arise
when documents contain errors, in either the date or the
age. U.S. censuses prior to 1850 pose additional problems
since broad ranges are the only indicators of household
members’ ages. Finding as many documents as possible
for individuals helps narrow the range of possibilities
for their birth dates.
You can use date calculations to produce two types of
results, a specific date or a date range. Some examples
will be used to help illustrate these calculations.
CALCULATING SPECIFIC DATE
Death certificates and tombstones are usually the only
documents that specify a person’s full age. The calculation
for a birth date requires a date of death and the full age
of the decedent in years, months, and days.
Subtracting the full age (years, months, days) from a
date is relatively straightforward. However, when the calculation
requires arithmetic borrowing, remember it is not decimal.
For example, Henry Edmund Bulson died on 17 July 1868 at
the age of 16 years, 10 months, and 13 days.1
To calculate his birth date, the subtraction problem would
To calculate months, you cannot subtract 10 from 7 without
borrowing. So you would borrow a year and add those 12 months
to the 7 months, as follows:
Henry Edmund Bulson was born about 4 September
When using a calculation to determine a birth date, always
show it as “about 4 September 1851”
or “circa 1814 to 1818.” Even when
the result of the calculation is a specific date, until
you can confirm with other sources that the person was born
on that date, it remains only an approximation. When citing
the source of the birth date or range, you should state
that it was calculated and include the source of the data
used in the calculation.
Another handy option for calculating a specific birth
date is the 8870 Formula.2 This formula is based
on a 30-day month, but there are nuances that you should
know to make it as accurate and simple as possible.
Using the 8870 Formula requires formatting the death date
as YYYYMMDD and subtracting the age in the format YYMMDD.
(For single-digit months or days, precede the digit with
a zero as shown for July (07) in the first example above.)
From the difference of these two values, you would subtract
a constant. For most of the calculations, this constant
Let’s look at an example. Catharine Rose Kanski
died on 8 December 1892 at the age of 81 years, 13 days.3
Because there are no months given in the age, you must borrow
one of the years and use 12 for the months.
Catharine was born about 25 November 1811.
Sometimes when you subtract the constant, the resulting
numbers do not look like recognizable dates. For example,
if the days come out to 43, you need to do more arithmetic
to subtract 30 days and increment the months. Likewise,
if the months are greater than 12, you adjust them and add
one to the year. To avoid this extra math, you can look
at your initial difference and tell which parts of the 8870
constant to use or to disregard the constant.
Martina Wilkins Van Orden died 13 March 1887 at the age
of 71 years, 4 months.4 Having no days for the
age is acceptable. No borrowing is necessary. The first
step of the calculation produces the following:
When you look at the difference, the months are 99 and
the days are 13. The 13 days is less than 30, so you do
not need to adjust them. The months, however, need to be
adjusted. You need only to subtract 88 from the months and
leave the days alone. That is why the constant used for
this calculation is 8800.
If you have a difference where there are too many days,
but the months are within normal range, then the constant
would be 70. In the case where both the months and days
are normal, the initial difference is the correct date and
no constant needs to be subtracted.
In 1582, when there was a transition from the Julian to
the Gregorian calendar, a number of days were dropped to
adjust for prior discrepancies. Adoption of the Gregorian
calendar did not occur simultaneously across the world.
In the case of the United States (then a British colony)
the adoption took place in 1752. Eleven days between 2 September
and 14 September were dropped from the year. The “8870
Formula” calculations work properly so long as the
initial date and the answer are in the same calendar. If
they cross the divide between Julian and Gregorian, results
will be inaccurate unless you subtract 11 days to adjust
the birth date.5
CALCULATING DATE RANGES
When an exact age is unknown, the calculation for birth
date will result in a range of dates.6 A single
document will, at best, provide a calendar year for a range.
As a simple example, if a child is shown as one year old
on 8 September 1928, you will have a full year of possible
dates for his birthday. He could have turned one year old
that day, or he might be turning two on the next day. The
range of possible birth dates is from 9 September 1926 to
8 September 1927.
Collecting many documents stating a person’s age
should narrow the range of dates because of the overlaps
of the ranges. The following three documents give specific
dates and ages for Daniel Bolson.
On the 1880 U.S. census (official date 1 June 1880) Daniel
is shown as 63 years of age.7 He might have turned
63 that day (b. 1 June 1817) or might be turning 64 the
next day (b. 2 June 1816.)
The census takers were told to record the age of the person
as of the official census date (not the date they visited
the household). The following table shows the official U.S.
decennial census dates.
(From 1790 through 1820, the official census day was the
first Monday in August.)
A Declaration for an Original Pension for a Father or
Mother dated 19 December 1884, gave Daniel’s age as
68. He might have turned 68 that day (b. 19 December 1816)
or might be turning 69 the next day (b. 20 December 1815).
In a letter to George B. Raum, dated 27 September 1890,
Daniel stated “in Cornwall ware I have lived 65 out
of 74 [years].”) He might have turned 74 that day
(b. 27 September 1816) or might be turning 75 the next day
(b. 28 September 1815). 9
The overlap of the dates in the three documents is 2 June
1816 to 27 September 1816, a range of approximately four
Seek out every possible document to help narrow the birth
date range for an individual. These techniques will help
you calculate the date or date ranges. If you find a document
and the calculated birth date range falls outside of other
ranges for that individual, the record may not pertain to
your ancestor but to someone with the same name. This should
help you keep your proper ancestors on your family tree.
1 Henry Edmund Bulson tombstone, Mt. Rest Cemetery,
Stony Point, Rockland County, New York.
2 M.J. Bisbee, CGRS, “Formula 8870,”Genealogical
Helper, 30 (March 1976): 80-81.
3 Catharine Rose Kanski cemetery record, 8 December
1892, The Green-Wood Cemetery, Brooklyn (Kings County),
New York. Certificate supplied 29 August 2001 by Jane Cuccurullo,
citing Grave 2467, Lot 27263, Section 135.
4 Martina Wilkins Van Orden death certificate
no. 375913 (1887) Manhattan, Municipal Archives, New York
City, New York.
5 For information on calendars, see Kenneth L.
Smith, A Practical Guide to Dating Systems for Genealogists
(self published, 1983).
6 For additional information on date range calculations,
see Roger Allan, “Birthdate Spectrum Overlaps,”
Family Chronicle Volume 2, No. 3 (Jan/Feb 1998),
7 Daniel Bolson household, 1880 U.S. census,
Orange County, NY, population schedule, Cornwall, page 83,
dwelling 167, family 193; NARA T9, roll 910.
8 Daniel Bolson, Civil War Dependent’s
Pension Application no. 322,375 on service of William H.
Bolson (private, Co. E, 176th New York Infantry), Record
Group 15, Records of the Veterans Administration, NARA.
Amy Larner Giroux, CGSM
This article was originally published in OnBoard,
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