Have a number increment automatically and Auto fill the cell if over 90

I have a need to have a number increase every day automatically and turn the cell to red fill after 90 days then if changed to a number less than 90 to un-fill the cell.
Example… I have to look up MAC addresses of phones and after 90 days I can delete the phone ext if unused. I know I could just put in the date but I want it to just display the number of days and red fill if 90 or over.
Thank you for any help you can provide.

What I am trying to accomplish is have the Inactive Days increase from the date entered. I think I figured out the auto highlight, it works works at home, I have to retry at work.
Example on Row 5, on 11/17/2020 I entered that it was Inactive for 238 days. If I come back and check it on 6/23/2021 it still says 238 and not 456 days.

Also is there a way to put colons in between every 2 characters. Right now I am using =CONCATENATE(MID(A1,1,2),":",MID(A1,3,2),":",MID(A1,5,2),":",MID(A1,7,2),":",MID(A1,9,2),":",MID(A1,11,2))but if I
try to copy the MAC it copies the formula.
I would like to be able to copy and paste in a MAC and it automatically put in the colons i.e. 2F:E2:21:3B:88:24

I apologize if my description is confusing, but I am not a Excel Guru. I wish I was.