Need formula to calculate value in multiple column divided by value in other cell (date )

Hey, need your help.

Trying to find an easy way to calculate Average CS sold per month based on total of column C to N (per line) divided by number of days represented by date entered in cell P4.

Formula in Cell O for each line is SUM of C to N.
I need formula in Cell P to take the total in Cell O divided by date entered in Cell P when I update my report.

I currently pull my sales report every two weeks and I manually change the formula in Cell P for each line to =+O6/1 (for example now for Jan). But if I pull my sale report lets say on Feb 15th, I would manually change the formula to =+O6/2.5.

Is there an easier way to calculate this based on the date I populate in cell P4?

Thanks!

Thank you for the formula and correcting the logic of ''middle of February'' for my calculation.
Greatly appreciated!