E1 = "Contracts Discharged"
E2 = C2 - C3
pull down to E29

H2 =SUM(E2:E29)
Total Contracts Discharged in Period
(check: is this equal to total starting number of contracts? Yes => none extend past check period)

H3 =SUMPRODUCT(A2:A29,E2:E29)/H2
Weighted Average Lifespan of Contracts in Period

Multiply how many contracts are discharged in a period by what year they're discharged in and sum up to get the total number of contract-years; then divide by number of contracts to get the weigthed average.

By the way, the result I'm seeing there is 2.1815 years.