I need my macro to calculate the date difference between 2 ranges in 2 different sheets.

Ive got the codes below using record macro, however my range is not fixed as i'm using unfixed cells like (Cells (b, c)).

Code:
ActiveCell.FormulaR1C1 =
        "=DATEDIF(R[5]C,'production breakdown'!R[-5]C[1],""d"")"
I've tried
Code:
Cells(b, c) = DATEDIFF("d",(Cells(b,y)),'Production
...