You can try this: right click the sheet tab, select View Code and paste in
Rich (BB code):
Private Sub Worksheet_Calculate()
Dim c As Range
For Each c In Range("D2:D200") 'modify the range to suit
If c.Value = 1 Then
Change the range in red to the range where the countdown days are.
This code will run automatically each time the sheet calculates. What I'm not sure of is whether a change in the system date will cause the sheet to recalculate.