Monthly Return Calc in Excel

I know this one will be super easy for someone....

Here is my data structure:
Column A: has the date for each month(11/30/00, 12/31/00) for many years, and the most current date typd in the next cell in that column.
Column B: the net asset value corresponding to each date.

Here is what I am trying to do: I want a formula in 1 cell that will calc the monthly return:
(Ending Value-Beginning Value)/Beginning Value
And, I want this formula to update automatically each month...

Thanks for your help.
RRA