The Dilemma of Calculating Birth Dates from Age at Death

One would think calculating a birth date from an age at death would be quite simple. Even if one isn’t a math pro, it’s easy to arrive at a birth date using one’s computer. The calculator in Windows even has a Date Calculation option to determine dates by adding or subtracting, such as finding a birth date by subtracting the year/month/day of age at death from the death date. Easy-peasy, right?

Well, not exactly. You see, there are several problems when determining a date by subtracting from (or adding to) another date.

One calendar pitfall is when the dates are prior to the 1752 calendar change. I’d learned about this as a teen starting into genealogy. To this day, my brain still does backflips, thinking about this calculation. So I resort to an online calculator that specializes in Julian-Gregorian calendar conversions.This lets me know that when my Loy immigrant ancestor arrived on 26 September 1741, while the Colonies were using the “Old Style” Julian calendar, the “New Style” Gregorian calendar we use today would put the arrival as Saturday, 7 October 1741. Dealing with converting one calendar version to another is enough to handle, but throwing in an additional computation between these calendars, such as determining a birth date from the subject’s age at death, is quite a bit to juggle.

Considering we’ve had the same calendar for 272 years, post-1752 dates might seem easier for calculating a birth date from death date and age. But here’s the second pitfall: there’s more than one method in calculating an age at death. One is by the calendar month, in which all the varying days in calendar months are counted. The second is by the 30-day method, in which all months are counted as having 30 days. Thus it’s critical, when calculating anything showing age at death instead of a birth date, to footnote which calculation method was used.

Last week I found an online 1932 funeral home record of Lena, 4th cousin of my grandmother’s father. It was great to find this, as her death notice was “short but sweet” without any mention of birth date or even parents/siblings. Despite several conflicting ages in prior census, ranging as much as a 20-year difference, the funeral home record contained her exact date of birth. That is, the handwritten month and year were clearly legible on the image, but the barely legible day might be 10 or 15. To solve the mystery, I used the Date Calculation mode on my computer’s calculator, which counted all the days in the calendar months. But the date result wasn’t a 10 or 15. Then I vaguely recalled the other method, not an option on this calculator. I ran the death date and age through an online calculator having the 30-day option, and it confirmed the day was the 15th.

I believe long ago I’d heard the 30-day method was used most. Of course, there’s always a chance the other method was used. This is why, in those days before birth and death certificates, it’s good to find other sources to back up a derived birth date. Such as an old newspaper account of the person celebrating his [fill in age] birthday on a certain day or a Bible record near the time of the birth. And, likewise, it’s a good idea to search for a newspaper obituary to confirm a gravestone death date.

After birth dates became commonplace on gravestones, gravestone dates still weren’t infallible. Such as I’ve found with the gravestones of several of my ancestors. This, I’ll detail in a future post.


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