Start of month (with date in A1)

=DATE(YEAR(A1),MONTH(A1),1)

end

=DATE(YEAR(A1),MONTH(A1)+1,0)

12 months ago from end of month subtract 12

=DATE(YEAR(A1),MONTH(A1)-11,0)

=AVERAGE(IF((A2:A1000=DATE(YEAR(A1),MONTH(A1)-11,0))*(A2:A1000<=DATE(YEAR(A1),MONTH(A1)+1,0)),B2 :B1000))


will average values in B2:B1000 where dates in A2:A1000 are greater than 12
months from end of month of date in A1 AND smaller than end of month with
date in A1

A1 can be replaced with TODAY() to use today's date

needs to be entered with ctrl + shift & enter



--

Regards,

Peo Sjoblom

Northwest Excel Solutions

Portland, Oregon




"gevans" wrote in message
...
I need formula that will determine a rolling 12 month average. I'm
starting
with individual days, so I need something that will recognise the start
and
end of each month, something that is date based rather than just count
based.
Any ideas?
Thanks in advance!
--
gevans