gnumeric spreadsheet newbie questions

I have some spreadsheets with about 30K rows each. In column A is a
time and date like this - 2017-11-01 00:01:16 UTC. The time increments
monotonically for a month then that's all the rows.

I'd like to sum the readings for each hour, The number of rows per hour
varies a bit with uneven intervals. So in a general sense:

set sum to 0
While hour is unchanged do:

	add readings in column B
end;

display hourly total in column C

and iterate over all rows until a blank row is found (EOF)

If there's some function to do this or some part of this I'd appreciate
a reference, or [perhaps hand-holding off-line.] The F1 help in my copy
of gnumeric 1.12.28 gives an error message.

Dave


-- 
In modern fantasy (literary or governmental), killing people is the
usual solution to the so-called war between good and evil. My books are
not conceived in terms of such a war, and offer no simple answers to
simplistic questions.

----- Ursula Le Guin