"Arvi Laanemets" wrote:
This assumes there is a single row for every moth,
and always a row for a month, does it?

Yes of course <g, the assumed set up was described
in the response, as always ..

More general solution (dates in column A, values in column
B, headers in row 1, sum is placed outside of data range, p.e. in cell C2)
=SUMIF(A:A,""&DATE(YEAR(MAX(A:A)),MONTH(MAX(A:A))-11,0),B:B)

Just a thought. The above would return zero if the dates were not "real"
dates.
(A possibility that I had guessed might be the case, in the earlier
response. It wasn't clear from the orig. post.)

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Rgds
Max
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