It took me a long time to wrap my mind around the idea of calculating birth date ranges from an age on a particular day but now that I understand how it works, I find it much more useful than just subtracting an age from a year and it isn’t that much harder to do.

Let’s use Frank M. Smith as an example. On 2 July 1885, he stated, under oath, that he was forty-eight years old. [1]

Subtracting 48 from 1885, I get 1837.

(To easily do this in my head, I subtract 40 from 1885 and get 1845. Then, because I can’t take 8 from 5, I take 8 from 15 and get 7 and just reduce the 10s by 1 to get 38 instead of 48. And, I try to remember to add things back up to double-check my mental math.)

So, if 2 July 1885 happened to be Frank’s birthday, he would have been born on 2 July 1837.

And, in fact, if he had been born any time between 1 January 1837 and 2 July 1837, he would have already turn 48 by 2 July 1885.

But what if he happened to have been born in the second half of the year?

If he was 48 on July 2, looking forward to celebrating his 49th birthday between 3 July 1885 and 31 December 1885, that would mean he was born in 1836.

So, the birth date range for Frank, who was 48 on 2 July 1885 is 3 July 1836 to 2 July 1837.

Let’s do one more example.

When Frank registered to vote on 16 October 1888, he stated that he had been in the city of Chicago for 52 years. [2] He was born there, so, theoretically, that would have been  his age.

1888 minus 52 equals 1836. So, the date range for Frank’s birth calculates to 17 October 1835 to 16 October 1836.

Now let’s look at these two date ranges together.

If — and that’s a big if — Frank’s age was stated correctly on the two dates, then comparing the two narrows his birthdate to between July 1836 and October 1836. Knowing that makes it easy to see that the exact birth date stated in a published family history falls squarely into the range of possibility. [3]

Some thing to keep in mind:

1) The approach is only as good as the information it’s based on.

2) Date ranges are particularly useful for comparing age information from records that were created in different months.

3) When calculating date ranges from census records, the census dates vary, depending on the census year. (For a quick reminder, just Google “u s census dates.”) Also, census takers might have recorded ages as of the census date or as of the date they visited–no matter what the instructions asked them to do.

And where can we find age information for particular dates? 

Quickly brainstorming from my experience with Chicago records, let’s see …

• marriage licenses
• parent information on children’s birth records
• death certificates
• voter registration (if the person was born in Chicago)
• proof of heirship interviews in probate files
• census records
• legal documents

So, in summary, here’s the way that I’ve finally wrapped my mind around the idea of calculating birth date ranges from an age on a particular day, month, and year:

Subtract the age from the particular year to get the end year. The end year is the same as the particular day.

Subtract 1 from the end year to get the start year.

The start day is the day after the particular day.

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1.     Cook County, Illinois, probate file no. 4-2790 (1885), Marcia M. Smith, proof of heirship, 2 July 1885; Cook County Circuit Court Archives, Chicago.