This isn't a question about any particular death certificate, but rather a general question. On modern birth certificates, say after 1950, the general style seems to be to give the date of death, the age in full years and then the date of birth. This seems quite logical.
However, on older death certificates, usually before 1900, the forms asked for the date of death and the age of the deceased in years, months and days. Why would this be done? Is there some benefit to this that I am not seeing?
Clearly, people in the 19th century did not wake up each morning telling their family "In case I die today, I am 79 years, 10 months and 8 days old." I assume that, like today, the relatives knew the birth date (or at least an approximation thereof), gave that to the town clerk and the clerk then manually calculated out the age for the certificate.
The obvious problem with this is that it introduces extra chances for error. In addition to a mis-remembered or misheard birth date, the clerk could make an error in arithmetic or when copying the answer over, and we as genealogists could make an error when re-computing the birth date (although I'm sure they weren't concerned with us at the time). And as I have found in my own research, arithmetic or transcription errors certainly occurred, as I have at least one death certificate from the transition period (about 1900-1950) where it asks for date of death, age in years months and days and date of birth, and the information is self-contradictory.
So given that making an error seems more likely with this practice, why was it done? Isn't the exact date of birth more important than the exact age? Or is that just a modern view?
It was pointed out to me in the comments that this might not have been a universal practice, so for clarity, I am from New England and most of my records are from New Hampshire and Massachusetts, with a smattering from Vermont and Rhode Island.
This is an example of the usual format, from NH, with the age in years/months/days and no birth date. The latest example of this style that I found from a quick scan through my images was from 1916.
And this is an example of a self-contradictory death certificate. It gives the date of death as June 5, 1912, the age as 89 years, 9 months and 13 days and the date of birth as August 23, 1822. However, if you use an online calculator and subtract the age from the date of death, you get a birth date of September 23, 1822.
As to how these dates and ages were calculated, I'm not entirely sure. Today there are formulas such as this one (http://mathforum.org/library/drmath/view/66857.html) that were used, but they may well have used a calendar. I don't honestly know. But I have over one hundred examples of death certificates and the vast majority give a calculated age, so I suspect they must have had a formula or method.
Of course, there is another potential wrinkle in all this. Suppose the deceased was born in New Hampshire in 1740 and died in 1820, living through the Julian to Gregorian switch. The clerk is given a birth date and calculates the age. He could calculate the right age. Or, the family could give him the Julian date and he could fail to account for the missing 11 days. Or they could give him a date that was already converted to Gregorian, but he could accidently convert it a second time, putting his result off by three weeks.
As you can see, the potential pitfalls of doing this are nearly endless.