# Convention for writing birth date inferred from exact age at death

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Many death records list an exact age at death in years, months, and days.  But including the number of months rather than just a total number of days into the year introduces a couple of ambiguities.

Example: John Smith died on 10 March 1850 and his death certificate reports his age as 25 years, 1 month, and 12 days.

If I subtract 25 years, and then 1 month, and then 12 days, I come up with the birth date

January 29, 1825.

The first ambiguity is that this assumes you start counting days with January 30 being number 1, January 31 being 2, etc.  But the reporter might view the birth date, January 29 as day 1, so our inferred birth date is off by 1 day and should really have been

January 30, 1825

(It's essentially the same issue as how you count years when the age at death is reported as "in the year of his age".)

The other ambiguity is more substantial.  If I take the death date 10 March 1850, subtract 25 years, then subtract 12 days, and then subtract 1 month (assuming birth date is numbered as the "zeroth" day) I come up with

January 26, 1825.

All I did is switch the order of subtraction of months and days and I produce a different birth date.  Compounding these two ambiguities, we could be off by as much four days from what the reporter intended.

Is there a convention for reporting inferred birth dates in a  range because of these ambiguities, like the 1750/51 convention for dealing with the ambiguity of old style and new style? Has anyone ever seen a death certificate that provides consistent instructions for computing age at death?  Is there any evidence that people have been consistent?
You would also have to figure in Leap Years!

Believe it or not, those signing the death records--the ones assigning yr, mo, days--used two different calculations.  As others have said, being off a couple days doesn't really matter much to our beloved departed or to the overall scale of a full life.  Just mark the date as "uncertain" and note in the bio that the date was calculated based on age of ____ at death on ___.
by Kathy Rabenstein G2G6 Pilot (243k points)
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Calculators like https://www.timeanddate.com/date/dateadd.html give the option of two different dates.
Very cool Ellen.  But I see it assumes everyone starts their numbering with early date being day zero.
The web utility allows you to add or subtract from the first date.
Why would you subtract from the first date?  You'd add from the first or subtract from the last, right?
Good question. I would bet there's not much consistency in how the calculations are done. I'd still mark it uncertain, but explain in your bio how you calculated the information. Nailing it down to a range of only four days is really pretty good, considering.
by Nelda Spires G2G6 Pilot (270k points)

I have used the formula I was taught, and like just about every date I've found for birth, marriage, death, and whatever else we find a date for, there's room for what you term as inconsistency ... I DO recall having found someone (2 or 3?) so far that were consistently credited with the same dates throughout all the documents I found. Surprised me. "Oh, like WOW, man!!" I remember BEING surprised, don't remember who they were, though.

Sometimes it happens, everything found is in agreement document after document. You just want to cry for joy.

BUT the Convention is that we make NOTE: of the ambiguities IN THE BIOG. That's the Convention. THE Convention.

PS/ NO ONE has ever put "Oh, WOW, like WOW, man, this information is Consistent from one document to the NEXT!! in the Biography.

by Susan Smith G2G6 Pilot (408k points)
I wrote a calculator script a long time ago (http://www.buckbd.com/genea/bdjscr.html), and I have a commentary on it (http://www.buckbd.com/genea/bdexpl.html). I did actually look at real data, but the results were equivocal. However, there were period prescriptions for the method, whether or not people followed them.
by Anonymous Buckner G2G6 Mach 5 (51.8k points)
Those of us born before the discovery of the wheel remember slide rules.  Those completing death certificates had something resembling that.  But you'd have to find the one used in a particular area to know what the particular method was.
Back when I first did that, I had to look everything up on microfiche, but a lot of those old texts are on Google Books now. This one has a pretty explicit method:

Very cool!  I'm a mathematician, and I love seeing historical texts like this.  I'd never heard of the term "compound subtraction".

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