You can use conditional formatting to achieve your goal. If you have the
original date in A1, and in B1 you have a formula =A1+90, then onditionally
format B1. Iso Cell Value is, use Formula Is, and insert the following
formula: =TODAY()<A1+80
Click on Format, select Patterns, choose Red, and OK out. Set the Cell
Background to Green, and it will remian green till 80 days after the date in
A1, when it will turn red.