Excel Countdown

I am trying to create a formula that will countdown days until next appointment due date in, lets say cell,A1.

In cell b1 I have the a 3 month appointment date, c1 a 6 month projected date and d1a 12 month projected date.

I have managed to create a formula that will do a countdown for each individually but ideally Id like one that catered for all.

For example, if todays date was greater than the 3 month date (b1) then they countdown would roll over and correlated with the date of the 6month (c1), and then continue to correlate with the 12 month date(d1) once the 6 month date was greater than today.

Thanks in advance.