Assessment time limits under section 153 govern reassessment, recomputation, and consequential orders after appellate or search-related proceedings. Time limits for completion of assessment, reassessment and recomputation under section 153 are prescribed by reference to the assessment year, updated returns, notice under section 148, and appellate or revisional orders. Different limitation periods apply to assessments under section 144, reassessments under section 147, fresh assessments after set-aside or cancellation, search-related proceedings, transfer pricing references, and orders giving effect to appellate or revisional directions. Special periods also govern revived search cases and consequential orders where verification or hearing is required.