My implementation of determining a person’s age at an event (see my blog post: Getting Ages Right) has led me to the conclusion that ages are weird. Normal mathematics does not work on ages, and they can lead you to the wrong conclusion.
For example, let’s say your grandfather died in 1980 and you know he was 80 years old. You’ve looked through all the 1900 records and can’t find his birth certificate anywhere. Maybe the solution is to look through the 1899 records as well.
The trouble is that a person is not said to have reached a certain age until they reach their birthday. So if your grandfather was born on June 15, 1899, then he would have reached his 80th birthday on June 15, 1979, and would have been considered to be 80 years old from June 15, 1979 until June 14, 1980 the day before his 81st birthday.
Working backwards from the date your grandfather died, say May 18, 1980. If you knew he was 80 years old, then he must have been born from May 19, 1899 to May 18, 1900. Someone surely would have mentioned if he died on his birthday, or if he was one day short of reaching his 81st birthday, so sometimes in your research you can get lucky and derive a likely birthday more accurately.
Because of the “birthday” requirement, we spend the first 365 days of our lives being 0 years old. We have not yet reached our first birthday. That is weird. If we could converse at a young age and someone asked us how old we were, we’d say “zero”. So what is normally done is that when we are less than a year old, our age is referred to in how many months old we are. We say the baby is 3 months old, or 9 months old. Three months old again has that weird meaning, being that it can range anywhere from exactly 3 months, to 4 months less a day. In the first couple of months, parents use “weeks old” and in the first week, they use “days old”.
On the day you were born, you were 0 years old. You were 0 months old. You were 0 weeks old. You were 0 days old. We can get down to hours, minutes and seconds, but they work the same and dates in genealogy are seldom quoted down to the time of the day. The one consideration are twins, with one born just before midnight on December 31, 1899 and the other born just after midnight on January 1, 1900, who just happen to be born on a different day in a different month in a different year.
So then there’s age arithmetic *sigh* which I’ve been having to deal with. If your grandfather was born in 1899 and died in 1980, how old was he when he died? A straight subtraction gives 81. But, we know he was 80. Someone born in 1899 could have been born from Jan 1, 1899 to Dec 31, 1899. If that person died in 1980, then that was from Jan 1, 1980 to Dec 31, 1980. The minimum age at death would have been with a birth of Dec 31, 1899 and a death of Jan 1, 1980 which would have been 80 years old (plus one day). The maximum age at death would have been with a birth of Jan 1, 1899 and a death of Dec 31, 1980 which would have been 81 years old (one day short of being 82). Therefore, the person had to be from 80 to 81 years old.
If a person was born “about 1899” and died “about 1980” then what do you do? In Behold’s case, it’s going to assume plus or minus 1 year on every date range. I’ll let you do the math as an exercise if you want to check me, but Behold will report the person would have been from 78 to 83 years old when they died.
There are also calculations of ages involving precise dates with approximate dates. Don’t worry about them. Behold will be reporting those age ranges as accurately as possible.
When events span a birth or the death due to approximations, these need to be reported. If your uncle married between in 1980, then you don’t want the age of your grandfather to be reported as 80 to 81. He died at the age of 80 and never reached 81. Instead Behold will report: age 80 to 7m after death. That information will be very useful to you, because if you find out that your grandfather was still living when they were married, you’ll be able to narrow down their date of their marriage.
I’m excited about the way these dates will be displayed in Behold. Getting them right is very tricky, but doing so will be very useful to everyone who uses Behold.