negative age

[jsnyders]

I noticed that a number of people have a negative age.
I calculate the age as
if to is 0
age is this year - from
else
age is to - from
In some cases this is caused by a 1 rather than 0 being used for the to date to mean still alive. In other cases it is because wikipedia gives the birth and death dates using two different calendars. The from date is using one calendar and the death date is using the other.