why do you want to add, say: 1.40, which is january 1940, to 3.59, which is march-1959?? the sum wil give 3.99 (march 1999) which is unusable...
Maybe you want to add the number of month only? not years...
place this formula in E1, and copy it to the last row
=IF(ISERR(MONTH(D1));0;MONTH(D1))
to extract the number of month from date
and sum =SUM(E1:E28) in E29 will give you the result of 35 total number of month for the sample posted