The closing date is the last day of your billing cycle. Your billing cycle is usually about a month long. The due date is typically 21 days after your closing date. So, if your closing date is on the 15th of the month, your due date would be on the 5th of the next month. The main difference between these two dates is that your closing date is the last day of your billing cycle, while your due date is the day that you need to have paid your bill by. If you don't pay your bill by your due date, you will typically incur a late fee.