I have been using the following formula to add the number of unique numbers
in column A.
=SUM(IF(FREQUENCY(CaseData!A:A,CaseData!A:A)0,1))

HOWEVER,

Now I want to only count unique numbers in column A, only if column C is
between 1/1/2005 and 3/31/2005 (or any 2 dates for that matter).

EXAMPLE:

A C
5123456 1/1/2005
5123457 1/5/2005
5123457 1/5/2005
5123457 3/7/2009

The result should be 2 - because there are 2 unique numbers in A while C is
between 1/1/2005 adn 3/31/05.