alright perhaps I am too much of a novice to understand

how do I do this?

1 2 3
a 1990 01 25 1993 07 21 =a2-a1 (displayed as "03 09 16")
b 1993 07 22 1999 04 29 =b2-b1 (displayed as "06 02 23")
c (sum a3+b3 displayed
as yyyy mm dd)


I have been working on this for a while and it seems impossible