It’s very simple:
For consumption between August 2nd and September 1st, the bill will be issued on September 1st, and the final repayment date will be September 20th;
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For consumption between September 2nd~~October 1st, the bill will be issued on October 1st, and the final repayment date will be October 20th;
October 2nd~~11th For consumption between the 1st of the month, the bill will be issued on November 1st, and the final payment date will be November 20th; and so on!