Countdown with Conditional Formatting

Hello,

I'm trying to show a countdown of business days to an employee's last day and would like to add conditional formatting as follows:

If the countdown is less than 0 days, just show 0

If the countdown is 3-0 days, bold in red font and highlight field yellow

If the countdown is greater or equal to 4 days, no need to do any conditional formatting

Can you please assist me with the formula and what field type it should be?

I was trying to do a formula along the lines of:

If(ToDays([Last day]-today())<=3,"

"<font color=red><b><div style=\"background-color:yellow;\"</div>"&([Last day]-today()),

([Last day]-today())")

But I know this won't take into account the 3 different formatting and I was getting error messages related to the field type.

Thank you!