I'd use a column just for the purpose, although you could use conditional
formatting to change a cell's shading/font color.
Lets say that the date the course has to be renewed by is in column A, then
this formula on the same row (row 2 in this example) should show you when it
is time to renew. Example assumes you can book up to 90 days early.
=IF(AND(A2>=TODAY(),A2<NOW()+90),"RENEW","")
the A2>=TODAY() part deals with courses whose renewal dates are in the past.
