Generate a bill 10. 1 1, and record the transactions in the period of 9.12-11.The final repayment date of the transactions during this period is10.

Generate a bill11.11,and record the transaction in the period of10.12-1.The final repayment date of the transaction during this period is 65438+.

And so on.

Obviously, 10. 1 1 has the shortest interest-free period, which is 20 days (10. 1 brush, 10.3 1 return), and the first day after bookkeeping is.