Narrowing birth dates — a free tool does the math for you

A census record offers you a moment in time when your ancestor was, say, five years old. That gives you a potential birth date range of 365 days, plus a possible Leap Day. If you use multiple records, however, you can use the overlaps to whittle down that range, getting closer to the real date. Doing this math in your head, unfortunately, can launch a headache. But I have a solution: the Date Narrowing Calculator. My gift to you.

The 365-day census birth date range

If we’re doing things the fast and easy way, we look at the year on a  federal census, subtract the age of our ancestor, and assume the difference is the likely birth year. Put “ca” in front of it, and you’ve covered for your imprecision. Fifty-fifty, you have the right birth year.

But we’re genohistorians. That’s not good enough. We are narrowing dates to get to real  numbers, if they can be had.

OBLIGATORY NOTE . . .
We all realize, of course, that the numbers are only as good as those providing and recording them. So, precision is an ideal we're drilling toward.

First, you need to know what date was being used to calculate the age — something called the “Census Day.” (For a great explanation, see The Census Day by William Dollarhide, written for the GenealogyBlog.) Regardless of what date the census taker arrived at your ancestor’s house, the age was being calculated based on the federally mandated Census Day, which changed from decade to decade. Here are the dates William Dollarhide reports:

Let’s say your ancestor Beatrice Mitchell shows up in the 1910 census as age three. Since it’s 1910, you know your census-taker — let’s call him Bob — was reporting Beatrice’s age as of April 15, 1910. Here are your outlying scenarios:

Earliest possible birth date:  April 16, 1906

If Census Day fell the day before Beatrice’s fourth birthday, Bob would record Beatrice as three years old. She was born one day shy of four years before — April 16, 1906.

Latest possible birth date: April 15, 1907

Let’s say Beatrice celebrated her third birthday on Census Day — perhaps inviting her gal pals in for a themed birthday party.  In this case, Beatrice’s birth date was exactly three years earlier — April 15, 1907 — and could not be later.

Narrowing dates using multiple records

Thanks to a census record, we have narrowed Beatrice’s birthdate to one of 365 days between April 16, 1906, and April 15, 1907 — more helpful than the normal “ca 1907.” That’s progress, but not where we want to be. While in our best-case scenario, we seek a quality record with her full birth date on it, we don’t always get that lucky.

Let’s add other similar records, those containing a date of the record and her age, and we can begin to squeeze that range. Narrowing dates with multiple documents can bring you within a few months or even days of the event.

Beatrice’s other records: further narrowing dates

In 1915, let’s say, a school report card dated May 30, 1915, indicates that Beatrice was 8 years old, which gives us a new birthdate range: May 31, 1906 – May 30, 1907. If you look for how that range intersects with our earlier dates, you see that the narrowing date range is now 319 days — an improvement :

May 31, 1906 – April 15, 1907 (narrower)

At age 13, Beatrice appears in the 1920 Federal Census, which had a Census Day of January 1, 1920. We now have a new range to take into the mix: January 2, 1906 – January 1, 1907. Intersecting that with the narrower dates above reduces the date range to just 215 days:

May 31, 1906 – January 1, 1907 (even narrower)

Finally, you find a marriage license record for Beatrice, dated August 4, 1931. She’s listed as 25 years old, adding a new date range of August 4, 1905 – August 3, 1906. We’re down to a 65-day window for Beatrice’s birth:

May 31, 1906 – August 3, 1906 (really narrow)

This new window tightens our research, allowing us to make a much better call about the other facts that surround Beatrice and her family.

A tool that makes this easy

In trying to do this sort of analysis on my own family history, I kept tripping over myself. Too much to remember. Too much to compare. Who’s going to remember the calculations?

That’s what spreadsheets are for, and I created one to take the pain out of this procedure. In my Date Narrowing Calculator, you enter each record, its date, and the age of your ancestor. You can use this for other types of event dates, as well — marriages, employment, anything that specifies years served or lived or married as of a particular date.

At the bottom of the Earliest Date and Latest Date columns on the right, the spreadsheet alters your range as you add records, so that you can see the narrowing dates. If one of the records you add does not fit with the ranges you already have, you’ll see a message in red to flag you. You’ll need to determine which records seem most credible.

If the record only has a month and year, you can choose a date within the month. Speculative, admittedly, but at least it brings you close to your needed range. You can also enter a fragment for infants — 1/12 for a one-month-old — and you will have a very tight range. (Note that this calculation can be off by a few days.)

If you are recording a federal census record, enter the census year in the appropriate column, and the calculator will look up the Census Day for that year. (It’s worth downloading the tool just for that.) Enter it in the date field, and let the calculator give you your range.

Download it free

Download this spreadsheet free from this Dropbox:

Narrowing Dates Calculator

*Disclaimer
I can't promise this tool will work on every system, with every combination of software. I offer it "as is," with a confidence that it will help most of you. If it is not working, I would like to hear about it, in hopes that I can improve it. But there are no guarantees. You load it on your (presumably virus-protected) system at your own risk. 

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