Google Spreadsheets : reference a cell at a variable offset or other formula

I want to be able to reference a cell whose address I will calculate using a formula that depends on other cells (i.e. "the cell that is X rows before this one, where X is read in D2"). How to do this ?

Here an example where I have a series of numbers, and I want to calculate the delta every 7. I can write "=A8-A2" manually but I would want to write something like "=A8-A[8-D2]". How to do this ?

(Of course the first 7 deltas are meaningless and therefore return errors, but that's no matter for now.)