Imported Data Creates Blank Cells That Aren't Really Blank

Here's what I'm attempting to do: For each column, X,Y, Z, I am attempting
to count nonblanks. However, the data was imported from Access and Oracle,
and Excel treats what appear to be blank cells as nonblanks. I've tested
this theory by highlighting a couple of "blank" cells and deleting them, and
my count changes. So, can I get Excel to put a value into my "blank" cells,
so then I could filter it out, or create a formula that would only count
dates in my columns (which is what I'm after).

This is what I'm looking at:

A B C
1 2/4/2006 2/6/2006 ("blank")
2 ("blank") 12/13/2005 1/7/2006
3 2/20/2006 1/15/2006 ("blank")

In each column if I use a COUNTA I'll get a total of 3, instead of 2 for A,
3 for B and 1 for C.