How calculate number of days based on a period

Summary:

Need to calculate the average revenue per month based on the number of days in the selected period. For example the January period has $100,000 of revenue. I need to be able to show the average revenue per day, so divide $100,000 by 31. If February was the selected period and had $90,000 of revenue I would need to divide the total revenue that month by 28.

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