>sorry - i meant in terms of using those dates in date calculations - i.e. - how many days between the birth of Charlemagne and the death of Abraham Lincoln.

Ken,

Date math fails here because the calendars are different. You would need to claculate the number of days from Charlemagne's birth to the adoption of the Gregorian calendar and then add the number of days from the Gregorian Calendar to the death of Lincoln.