Del. Code tit. 30, § 514

If the amount of the taxpayer’s federal income or estate tax liability reported on the federal tax return for any taxable period is changed or corrected by the Internal Revenue Service, or other competent authority, the taxpayer shall report to the Director such change or correction in federal tax liability within 90 days after the final determination of such change or correction and shall concede the accuracy of such determination, or state wherein it is erroneous. Any taxpayer filing an amended federal income, estate or gift tax return shall also file, within 90 days thereafter, the appropriate amended tax return under this title and shall provide to the Director such additional information as the Director may require.