Working Capital Help

hi on this question of MCQ from pilot paper The management of XYZ Co has annual credit sales of $20 million and accounts receivable of $4 million. Working capital is financed by an overdraft at 12% interest per year. Assume 365 days in a year. What is the annual finance cost saving if the management reduces the collection period to 60 days? The Examiners answer is Finance cost saving = 13/365 x $20m x 0·12 = $85,479 where is the 13 coming from ?