If your great-grandfather died 22 February 1922 at the age of 46 years, 7 months, and 23 days, how do you find out when he was born? Just plugging the numbers into your favorite date-calculating software may not be enough. Barbara Levergood's 25-page instructional article in the March National Genealogical Society Quarterly explains why, and much more.
His age at death may have been the result of a calculation, but we don't know what kind of calculation. "Such calculations are not primary information, may be incorrect, and may have been calculated using one of several methods," writes Levergood -- and different methods can produce different birth dates when we try to run the original calculation backwards.
It turns out that by taking up genealogy we didn't escape mathematics. Not all genealogical projects require us to determine every date to the day (and of course in any given case non-mathematical sources of error may overwhelm computation mistakes), but often proper technique can save a lot of trouble -- for instance,
- when we need to establish a likely range of dates within which to search for a vital record, or
- when we need to distinguish spouses or children over several censuses, or
- when a brief biography gives an approximate initial date for one event and gives other events as happening "three years later," "about ten years after that," and so on -- and we want to know how large the reasonable date range may have become by the end!
Members of the Association of Professional Genealogists will recognize this as a nice companion piece to Steve Morse's article on historical calendar changes in the March APG Quarterly. They're both keepers.
Barbara Levergood, "Calculating and Using Dates and Date Ranges," National Genealogical Society Quarterly 102 (March 2014): 51-75.