Sorry, but the number of days between 15 and 16 is

Sorry, but the number of days between 15 and 16 is one, not two, which appears to be were you made your error. When counting the number of days to an event you don't count the initial day (today, the 15th), but you do count the day of the event (the 20th). That's five days, not six. Ask any lawyer. Count it up, you say: 15 to 16 is one, 16 to 17 is two, 17 to 18 is three, 18 to 19 is four, 19 to 20 is five.