Because this post needed lots of diagrams, sections, and boxes, I did something unusual and hand-wrote it on graph paper and scanned it to a PDF. To continue reading, please click the link below.
Beware: you’ll have to do some thinking! This is a really common and surprisingly difficult problem, though, so it’s worth trying to understand.
Related to problem #3, I have frequently seen another mistake. I work a lot with family history. Occasionally tombstones will say that so-and-so died in the “__ year of his/her age.” For example, the tombstone of one of my wife’s ancestors says that Experience Loree died “Sep. 18, 1835 in the 73rd year of her age.” Many researchers conclude that she was 73 years old when she died. In fact, she was still 72 years old when she died, but was into her 73rd year. An easy mistake to make.