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Amy Larner Giroux, Ph.D., CG, CGL, “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 be:

Mo. Day
1868 07 17 Date
-16 10 13 Age

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:

1867 19 17 Date
16 10 13 Age
1851 09 04 Birth

Henry Edmund Bulson was born about 4 September 1851.

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 is 8870.

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.

– 8870

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:

– 710400

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.

Correct Date

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.

Calendar Transition

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.

Document 1
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.

Year Date
1790 2 August
1800 4 August
1810 6 August
1820 7 August
1830–1900 1 June
1910 15 April
1920 1 January
1930–2000 1 April

(From 1790 through 1820, the official census day was the first Monday in August.)

Document 2

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). 8

Document 3

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 months.

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), 49.
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.
9 Ibid.


Amy Larner Giroux, CGSM

This article was originally published in OnBoard, BCG’s educational newsletter and is protected by copyright. Individuals may download and print copies for their personal study. Educators are granted permission to provide copies to their students as long as BCG, OnBoard, and the appropriate author are credited as the source of the material. Republication elsewhere is not permitted.