Have you tried the Average A function?

"Gary''s Student" wrote:

Let's say your data is in column B from B1 thru B366, being built day-by-day.

In column A record the month ( 1 for January, etc.) then:

=SUMPRODUCT(--(A1:A366=1),--(B1:B366))/COUNTIF(A1:A366,1)
for Jan

=SUMPRODUCT(--(A1:A366=2),--(B1:B366))/COUNTIF(A1:A366,2)
for Feb

This will "grow" the averages. If you try to go too far into the future,
you will get zero divide errors.

--
Gary''s Student
gsnu200712