A few suggestions which may save effort next time you do this:

- Only use the $ symbol as much as necessary. Usually you only need one (e.g. $C12 or C$12 rather than $C$12), unless you're referencing a single input. In your example, the excessive use of $ on the 'Detail' tab prevents me from adding an extra row for an employee simply by copying-and-pasting

- SUBTOTAL can be better than SUM for summing columns of numbers. You want subtotals for Sales, Support etc., as well as a grand total. SUBTOTAL can be applied to a whole column of values without fear of double-counting, as it ignore existing SUBTOTALs.

- EOMONTH(x,1) will give you the last day of the month after the date x. EOMONTH(x,0)+1 will give you the first day of the month instead. It's a bit easier to read than your DATE(YEAR, MONTH+1, DAY) construction.