This worked perfectly! Amazing! Thanks tons!
There is no way I would have gotten to this on my own, now I need to figure out what you did so that should be able to do it myself next time.
The IF statement says that if a particular row in the array passes the logical test - which I'll explain in the next paragraph - then return the value from the usage column, otherwise return 0. So you get an array that's 19,999 cells, some filled with 0 and some with a usage value, and we sum that array.
The logical test works by 'multiplying' several logical tests by each other. The first parentheses creates a TRUE/FALSE array indicating whether each row is the month you're looking for. The second creates another TRUE/FALSE array indicating whether each row is the year you want (the one that contains the latest instance of the month you're looking for) - which I'll explain in the next paragraph. Multiplying the 2 arrays together results in 1 array that has TRUE only if both of the 'input' arrays are TRUE, and FALSE otherwise.
The year you want is determined by creating an array that's filled with 0 when the month is not the month you're looking for, and with the year where it is the right month. Take the max of that array, and you get the year that contains the latest instance of the month you're looking for.