Sales Run-rate. Too Tough For Me

I'm trying to calculate a sales run-rate which will change on a day-to-day basis, to predict the end-of-month sales total.

The invoice values are in the data range H17:H74 (I don't want the final formula to add up the refunds in this field i.e. negative values)

The date field is in data range C17:C74.

So basically the formula will need to add all invoice totals (excluding refunds) and divide by the current number of days worked in the month (not duplicating days in the date field) and then multiply this by the average number of working days in a month (21). This should should give a predicted end of month sales total.

Am I just making things up that are impossible to do on excel?

I've had a bet with the other guy in the sales office because I said excel can do a lot more than he thinks. He's under the impression that excel begins and ends with what they teach you at school.