I just received my bill today and it is already past the due date.

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You are probably looking at the "Bill Date" rather than the "Due Date" of your current charges. Residential bills are issued quarterly and are generally mailed on the last day of the month. The current charges are due on the 25th of the following month. On the right side of your payment stub there is a box with the Due Date for the Current Charges on your bill. If you have a previous balance, that balance is due upon receipt.