You can also have in the error message how many days are remaining instead of saying just "OK" or something, which is a bit more usefull...although can someone suggest how you work out how many days left when subtracting a date from a date, sure I've done it before but can't remember how!
I'd also look to add some indicators for conditional formatting, e.g. so anything with 10 days or more left is green, anything betwen 9 - 1 days left is amber, anything 0+ days overdue is red.
EDIT: think this works mate:
=IF(A3<TODAY()-21,"Overdue", SUM(TODAY()-A3))
Where A3 = the date being referenced, this will show you if less than 21 days how many days remaining, or how many days it is in the future between today and that day.