I need to write a formula which will search a range of cells containing dates
and if any of those is greater than todays date less 21 days, then return the
text "not due".

This is for a customer data base which has home page showing all customers,
and subsequent pages for indvidual customers. As contact dates are added for
each customer on their own sheets, then the home page should show if they are
due (or overdue) for a call