Charles SULLIVAN on the Kentucky Genealogy & History Discussion mailing list submitted this to the list because he thought it would be helpful.
This method of calculating date of birth is as follows:
In Todd's case: My 3rd great-grandmother Hester Jane GOLDEN, died in Bridgeport, West Virginia on November 9, 1863. On her headstone it states "Aged 36 years, 11 months, 19 days." To calculate:
RULE OF 8870
18631109 (Died: November 9, 1863)
18261129 (Born: 1826, 11 month, 20th day or November 20, 1826)
Three things should be pointed out:
1. The 8870 method assumes all months have 30 days and yields only an approximate date, although it may be exact in many cases. In this particular case, working backward from November 9, 1863 by first 19 days, then 11 months, then 36 years, yields the "exact" date of November 21, 1826, one day off from the 8870 answer.
2. The above notwithstanding, the 8870 method has been known for a long time, and "might" have been used by the person who calculated the age at death in the first place, so the "approximate" date may in fact be the correct date after all.
3. Humans are fallible, so none of the above certain, even though carved