This method of calculating the control character was devised by
mathematician Erkki Pale (1962) to detect input errors but also to
detect errors produced by early punch card machines. The long number
produced by writing the birth date and the personal number together are
divided by 31 and the remainder is used to look up the control character
from a separate table containing alphanumeric characters except letters
G, I, O, Q and Z.
The method of calculating the control character does not need century
character and therefore the function has an option to omit it.