you can - BUT it wont update , untill something changes on the sheet and it carries out a calculation otherwise i guess VBA which is constantly refeshing
I setup something like that back in the 90's for people to see on there screens when a threshold was reached for 20+ users
conditional formatting will change the colour based on the time entered and the time NOW() - but as i say - if the spreadsheet is just left open on a screen nothing will happen, until a change is made