I need to track the expiration of a specific period of time -- 6 months --
via a countdown cell.

Basically, I have a column (i.e., A1-A150) where I will be entering a date.
I will need to formulate another column whose cells (i.e., C1-C150) countdown
six months from the date entered in the corresponding row (i.e., C1 counts
from the date in A1; C35 counts from the date in C35).

Any advice on this would be greatly appreciated!

Some other questions:
1) Is there a specific format I should enter the dates in to ensure
countdown accuracy?
2) Is there a way, once the countdown period expires, to conditionally
format the cells so that they turn red, or even email me an alert? (The
former seems possible, not sure about the latter, though)
3) Is there a way to coundown this period so that it alerts me, say, ten
days before the six month period elapses?

Thanks so much in advance!