how to calculate number of days in a month based on key date variable?

Hi Folks,

We are developing a query DPO (Days payable outstanding report) in that to calculate

DPO the formula is (Accounts Payable+Provisions/Cost of Sales) * No of Days.

so, here based on key date which we enter we need to get no of days dynamically & calculate no of days.

Appreciate your valuable inputs for solving this.