If I have a column of dates (column A) which are in order but not sequential
and a corresponding column of values(column B), how can I calculate an
average for the values dating back one year from the last cell.

Column A Column B Column C
6/1/97 5 5/1=5
10/3/97 10 15/2 = 7.5
5/7/98 3 13/2 = 6.5
10/4/98 78 91/3= 30.3
etc