I'm trying to work out how to calculate the number of days a person has taken
off in a running twelve month period. For example if a person had been off 2
days in May 06, 3 days in December 06 and 10 days in April 07. The 12 monthly
total at the end of April 07 would be 15 days. However at the end of June 07
it would be 13 as May 06 is now outside the 12 month period