How old are you? This question would appear to have just one correct answer, an answer usually expressed by adults in years. Those who enjoy a good mathematical puzzle might try to express their age in days—also a unique and determinable answer. But what if you were asked to express your age in years, months, and days?
Let's say I was born March 20, 1964 (I was not, actually, and my Social Security number is not 004-76-3893). Today is July 18, 2005. In two days, I will be 41 years, 4 months, and 0 days. Today, I am 41 years, 3 months, and . . . how many days? Counting back to June 20, I find the answer is 28 days. But this is not the only possible answer. If I count back three months, and then count back the days to March 20, I find the answer is 29 days. Since March and June have 31 and 30 days respectively, the order in which I perform my calculations changes the outcome. Look at it another way: I subtract 1964 from 2005, and get 41 years; I subtract March (3) from July (7) and get 4 months. In order to subtract 20 from 18, I must "borrow" a month. But do I borrow a month of 31 days, or of 30 days? There is no single correct answer.
On old death records and gravestones, the age of the deceased was often given in years, months, and days. Whoever recorded the death, or carved the gravestone, probably had two dates in hand—the dates of birth and death—and used these to calculate age at death. But, as we've seen, what age results may depend upon the process by which it was calculated.
To make matters worse, some clerks adopted a "30-day month" policy. When calculating age at death, they assumed that all months had 30 days. This makes it easy for us genealogists to figure out the date of birth, but only if we know that this was the clerk's policy, and only if he followed this policy correctly and without exception. (There are also rare reports of a "28-day month" policy, to which a similar caveat (and a few others) would apply.)
Unfortunately, a death record or gravestone which bears only the date of death and age cannot be considered a reliable source for date of birth. (Death records should always be considered secondary sources for dates of birth, except in special cases, e.g. stillbirths.) Such sources, though, can be useful in confirming a date of birth found elsewhere, or confirming that a birth record and death record refer to the same person. If a calculated date of birth is the only birth date you can find for an individual, don't fret. Include it in your genealogy, but explain in detail the source of the date, and how you arrived at it.