Since you want a countdown, you'll want to subtract today's date from that date
that is 6 months in the future.
If A1 contains the start date, then you could have the date 6 months from there
in A2:
=date(year(a1),month(a1)+6,day(a1))
(format it as a date)
Then in A3, you could have the countdown formula:
=A2-today()
(and format it as General (not a date))
You could elimnate those helper cells as much as you wanted:
A3 could contain:
=date(year(a1),month(a1)+6,day(a1)) - today()
or even embed the date into the formula and drop the reference to A1:
=date(2009,12,25) - today()
(where Dec 25, 2009 is that date in the future.)