Revised return and due date rules are recast for different assessee categories, updated returns, and loss-related filing conditions. Section 263 is amended to prescribe different due dates for assessees based on audit status, section 172 applicability, and other taxpayer categories. The provision on revised returns is substituted to permit correction of omissions or wrong statements within twelve months from the end of the relevant tax year or before assessment completion, subject to section 428(b). The updated-return rules are also modified for loss cases and returns filed pursuant to notice under section 280, with related cross-references correspondingly adjusted.