Let's try building a formula
If we could identify the relevant cells then the average would be
=SUM(TheCells)/COUNT(TheCells)
Because the start time and end time can be in the middle of a row it is
a bit complicated to identify the cells.
I would go for
=(SUM(TheDaysCells)-SUM(BeforeStartCells)-SUM(AfterEndCells))/COUNT(The
Cells)

where TheDayCells are all the readings for the days concerned from
StartDate to EndDate inclusive,
BeforeStartCells are those cells in the row for StartDate which are
before the start time
AfterEndCells are those cells in the row for EndDate which are after
then end time.

I am assuming that the meteo. data has one row per date and that the
dates run sequentially. Name the first date in this table "FirstMet"
If you also name the columns StartDate, StartTime, etc. it will help
make the formula comprehensible.

StartDate-FirstMet is the row offset from the FirstMet cell to where
the data starts, and StopDate-StartDate+1 would be the number of rows
involved. So, TheDayCells would be
OFFSET(FirstMet,StartDate-FirstMet,2,StopDate-StartDate+1,24)
BeforeStartCells would number INT(StartTime*24)+1
(so that a StartTime of 8:45 would ignore the first 9 times, from 0:00
to 8:00) so BeforeStartCells would be that many cells starting in
column 3:
OFFSET(FirstMet,StartDate-FirstMet,2,1,INT(StartTime*24)+1)
AfterEndCells would start at column INT(StopTime*24)+4 (so StopTime
02:30 would start ignoring the cells from column 6) running to the end
of the 24 cells in the row for the StopDate, so AfterEndCells would be

OFFSET(FirstMet,StopDate-FirstMet,INT(StopTime*24)+3,1,23-INT(StopTime*
24))

So the total of the meteo readings would be
=(SUM(OFFSET(FirstMet,StartDate-FirstMet,2,StopDate-StartDate+1,24))-SU
M(OFFSET(FirstMet,StartDate-FirstMet,2,1,INT(StartTime*24)+1))-SUM(OFFS
ET(FirstMet,StopDate-FirstMet,INT(StopTime*24)+3,1,23-INT(StopTime*24))
))

and so the average will be
=(SUM(OFFSET(FirstMet,StartDate-FirstMet,2,StopDate-StartDate+1,24))-SU
M(OFFSET(FirstMet,StartDate-FirstMet,2,1,INT(StartTime*24)+1))-SUM(OFFS
ET(FirstMet,StopDate-FirstMet,INT(StopTime*24)+3,1,23-INT(StopTime*24))
))/(COUNT(OFFSET(FirstMet,StartDate-FirstMet,2,StopDate-StartDate+1,24)
)-COUNT(OFFSET(FirstMet,StartDate-FirstMet,2,1,INT(StartTime*24)+1))-CO
UNT(OFFSET(FirstMet,StopDate-FirstMet,INT(StopTime*24)+3,1,23-INT(StopT
ime*24))))

I bet you are glad you asked!



Bill Manville
MVP - Microsoft Excel, Oxford, England
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