your first two entries, which makes 9.98, as you say, came from:
6.63 +3.35, right?
But:
6.63 is just what you choosed to SEE on screen, the number behind 6.63 is 23163, same for 3.35, the number is 12844, you can format this number to be shown in many ways, but the number behind what you see is the same.

You have to read more about excel date/time format.
A good start could be this:
http://www.cpearson.com/excel/datetime.htm
anyway, if you insist to do this type of calculation, you have to extract month and year (last 2 digits, and transform it as decimal number, in order to add it to month)
(Why on earth do you need this??)
the formula in E1, copied down , is:
=IF(ISERR(MONTH(D1));0;MONTH(D1))+VALUE("0,"&IF(IS ERR(YEAR(D1));0;RIGHT(YEAR(D1);2)))

and the sum is 37,12 , not 26,83

Attached Files

Copy of 12.xlsx-2.xlsx (11.2 KB, 10 views)