Comparing changing data in 5 excel sheets


I have a daily spreadsheet that I need to compare to the previous day and so on for the wk. In line A i have a list of products,in line B there bestbefore date and in C I have there code life. I need to compare how much code life the products have each day to see if it changes by more or less than 1 days. However as the products sell out or we receive more stock in, the columns day to day may change. For instance I may have 3 excel rows of bbq sauce today but as we have received a delivery overnight the data may tomorrow be 5 rows, 6 rows of chicken today but only 2 tomorrow as we sold some, and so on with each product. I tried using a IF formula but that only returns if data is continually the same each day.!. Can anyone help.