Limitation period computation clarified to include exclusions or extensions under provisos to section 149, adjusting assessment timelines. Amendment to section 151 deletes the phrase concerning the absence of a Principal Chief Commissioner or Principal Director General in clause (ii) and inserts a proviso that the period of three years for clause (i) shall be computed after taking into account periods of limitation excluded or extended by the provisos to sub-section (1) of section 149.