Friday, May 9, 2014

A "Calculating" Post


How many times have you calculated a date of birth from a census record? If you’re like me, you’ve done it countless times. But have you ever considered how you do it and the importance of recording that in your research notes? A recent article in the National Genealogical Society Quarterly[1]
has shown me that my usual method is probably not “best practice.” Instead of simply subtracting the given age in a census from the census date and entering it into my database preceded by “calc,” I am now more likely to enter a date range and make a research note on how I calculated it.

The article describes four methods of date calculation and the reasoning behind them. I won’t go into the details of those methods here—you can read the article yourself—but it did get me thinking about dates and how they are calculated. Which method was used to calculate the dates?  Was the age simply subtracted from the year of the census? Or was the age subtracted from the official census date? Maybe it was subtracted from the enumeration date? If we clearly specify how we calculate our event dates, our research is deemed more credible and others can follow our reasoning process. We also advance our own research by narrowing or expanding the time frames in which we search for records.

After reading this article, I decided to try one of the methods to calculate a birth date from a death date and age at death in order to find a birth record. No surprise; but it helped me find a record I may have otherwise overlooked.

My grandfather’s little brother, Harry, died on 7 Sep 1901 when he was just 8 months old. It’s unlikely that he died at exactly 8 months; the number of days was probably just not recorded. Originally, I had counted back eight months and guessed he was born “abt Jan 1901.” The calculator feature in Legacy produced an exact date of 7 Jan 1901. Obviously, those unspecified number of days were not accounted for by either of the methods I chose.

A search for Harry’s birth record produced a possible record dated in December 1900, but not the January 1901 date I was looking for. What follows is the detailed method of date calculation I used in order to expand the time frame of the date of birth for Harry using a reference date (his date of death) and an interval (his age at death).

First I started with the reference date written from largest to smallest units using Arabic numerals—1901-9-7. The year is 1901; September is the ninth month; Harry died on the seventh day. This date will be used to start my calculations for both the bottom date range and the top date range.

            Bottom Range                         Top Range                   Steps
1.         1901-9-7                                  1901-9-7                      reference date

I then entered my interval of 8 months; but first I needed to put it into the proper format. Using the letter x as the unknown, Harry’s interval of 8 months became 0 years, 8 months, x days, or 0-8-x. That interval is entered into the “Top Range” column.

Bottom Range                         Top Range
1.         1901-9-7                                  1901-9-7                      reference date
2.                                                              0-8-x                       interval

Then, one (1) is added to the interval to the number that is the farthest right, and placed in the column labeled “Bottom Range.” Since the number in the farthest right is 8, add 1 to get 9. This is the adjusted interval for the bottom range.

Bottom Range                         Top Range
1.         1901-9-7                                  1901-9-7                      reference date
2.                                                              0-8-x                       interval
3.               0-9-x                                                                      adjusted interval

Sometimes there is a need to “borrow” in order to complete the math calculation. This happens when the interval number is equal or greater than the number of months (or days) in the reference date. Notice that this is the case in the bottom range column. So borrowing one year from 1901 gives me 1900; adding the borrowed year as 12 months to the number of the month (9 for September) gives me 21 months. This becomes the adjusted reference date for both columns.

Bottom Range                         Top Range
1a.       1901-9-7                                  1901-9-7                      reference date
1b.       1900-21-7                                1900-21-7                    adjusted reference date
2.                                                                 0-8-x                    interval
3.                 0-9-x                                                                    adjusted interval

Next, subtract the interval and the adjusted interval from the adjusted reference date. For the bottom range: 9 months from 21 months gives me 12 months; for the top range: 8 months from 21 months gives me 13 months.

Bottom Range                         Top Range
1a.       1901-9-7                                  1901-9-7                      reference date
1b.       1900-21-7                                1900-21-7                    adjusted reference date for bottom range
2.                                                                 0-8-x                    interval
3.                 0-9-x                                                                    adjusted interval
4.         1900-12-7                                1900-13-7                    subtract

Now, add 1 day to the bottom range so the 7 days becomes 8 days.

Bottom Range                         Top Range       
1a.       1901-9-7                                  1901-9-7                      reference date
1b.       1900-21-7                                1900-21-7                    adjusted reference date for bottom range
2.                                                              0-8-x                       interval
3.               0-9-x                                                                      adjusted interval
4.         1900-12-7                                1900-13-7                    subtract
5.         1900-12-8                                1900-13-7                    add 1 day to bottom range

Finally, write out the newly calculated dates. Normalize the date in the top range (we don’t have a 13th month), by giving back the 12 months to 1900, making it 1901-1-7.

Bottom Range                         Top Range       
1a.       1901-9-7                                  1901-9-7                      reference date
1b.       1900-21-7                                1900-21-7                    adjusted reference date for bottom range
2.                                                              0-8-x                       interval
3.               0-9-x                                                                      adjusted interval
4.         1900-12-7                                1900-13-7                    subtract
5.         1900-12-8                                1900-13-7                    add 1 day to bottom range
6.         8 Dec 1900                              7 Jan 1901                  write out dates and normalize

The newly calculated date range for Harry’s birth is now between 8 Dec 1900 and 7 Jan 1901. This has expanded on my estimated date of “abt Jan 1901” and Legacy’s calculated date of 7 Jan 1901. When I compare this date range to the birth index record search, the December 1900 birth date looks more promising. This method, with some slight adjustments, can also be used to calculate later event dates and ranges, such as using a tombstone inscription to calculate a death date from the date of birth and age of death in years, months and days.

Ms. Levergood suggests that we account for all possibilities by borrowing 31 days, calculating the results, and then subtract 3 days to create the bottom end of the date range. The 3 days account for the months with varying numbers of days, including leap year. One important point to remember is that these methods of calculation assume that all dates and intervals are based on the Gregorian calendar with the year beginning on January 1.

These methods of calculation will help me to more accurately search for records documenting the lives of my ancestors. Of course, I will remember to record the method used in my research notes. Who knows what new clues I may find?





[1] Barbara Levergood, “Calculating Dates and Using Dates and Date Ranges,” National Genealogical Society Quarterly 102 (March 2014):51-75

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