=AVERAGE(IF((A1:A100=MAX(A1:A100)-365)*(A1:A100<=MAX(A1:A100)),B1:B100))

which is an array formula, it should be committed with Ctrl-Shift-Enter, not
just Enter.

--
HTH

Bob Phillips

(replace somewhere in email address with gmail if mailing direct)

"Hunter" wrote in message
...
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