Creating an excel function which uses index functions (or something similiar)

Hello All,
I'm trying to build a function that does the following and i just cant get it to work.

The function takes todays date, and a contract start date)
It then uses the contract start date to search in an array for a corresponding price in the cell next to it.
It then adds 1 month to the contract price and finds the adjacent figure for that cell.
Those two prices are then multiplied and divided by the sum total of the number of days in those months)

I've done this with in cell functions however, i want to understand how it would work in my own function.

Everything i've built doesnt seem to work and i think its because i dont understand how to find the adjacent cell properly or how to add a month to a date thats defined.

Thanks for any help