1. Your R[-1]C references refer to the cell one row above in the same
column as your formula, when your earlier post implied that the buy
date was in the same row. If your buy date is immediately to the left
of the cell containing this formula, you should use RC[-1]. Maybe the
cell above contains a date in this month, which would give you the zero
result. Ensure that you are referring to the cell with the buy date
rather than to another cell. Otherwise, you will continue to get a
strange result. Can you use the default reference notation, A1, B1
etc.? You may find it easier to use.

2. You are still subtracting today's date from the (earlier and
therefore smaller) buy date which will give you a negative result, as
Mike pointed out. You should subtract the buy date from today's date
to get a positive result.

If you get the elapsed time correct, your basic growth rate formula
should give you a reasonably accurate result.