Formula to copy and replace values from one cell to another

I'm not an excel expert, in fact I just know the basics to get by. We have a stock report that stores best before dates and quantities among other information. It displays best before date, number of days shelf life left, quantity good stock and quantity short dated stock each in its own cell. Is there a formula to move good stock quantity to the shortdated stock cell if the number of days left reaches a certain total?