Cash conversion cycle
Days sales outstanding plus days inventory outstanding, minus days payable outstanding: how long cash is tied up in customers and stock after counting how long the firm takes to pay suppliers.
The first two clocks are the operating cycle. Subtracting payable days asks how much of that cycle the firm still has to fund itself. A longer cycle consumes working capital. A shorter one releases it, which is why a rise in working capital is subtracted when profit is turned into free cash flow.
Negative is allowed. When suppliers are paid after the cash from the sale is already in, CCC is below zero and the payables are funding the loop. That is a source of cash, not a bug in the formula.
The cash conversion cycle calculator adds the three days.