I have a column of numbers, that grows one cell per day. I need to
find the difference between the previous day's cell and today's cell.
... Is there an IF function or other
formula that can be used in the column that would would look only at
the last cell with data and the previous to it, ignoring the rest and
then would change as data is entered into the blank cell infront of
it?


One way is to use something like:
=OFFSET($A$1,COUNT($A:$A)-1,0)-OFFSET($A$1,COUNT($A:$A)-2,0)
This assumes there are no gaps in the column.