Del. Code tit. 30 § 5115
When any distributor fails to file monthly reports with the Department of Transportation as required by § 5113 of this title or when such distributor fails to pay the Department the amount of taxes due to this State as required by § 5114 of this title, a penalty of $5.75 per business day shall accrue up to a maximum of $28.75 for each report. For each report filed more than 5 business days late, the penalty shall be $28.75 or 12 percent of the tax due, whichever is greater, for each such report. Any tax due shall also bear interest at the rate of 1 percent per month, or fraction thereof, until same is paid; however, the Department may waive all or any part of the penalty and interest when it is established to the satisfaction of the Department that failure to file the monthly report or pay the tax by the twenty-fifth day of the month was not with intent to violate the law.
A month, for purposes of calculating interest in this chapter, shall be from the twenty-sixth day of a given month through the twenty-fifth day of the following month.
30 Del. C. § 5115