The first proviso to section 149(1)(b) embodies a negative command restraining the Assessing Officer from issuing a notice under section 148 in respect of an assessment year prior to 01.04.2021, if the period within which such a notice could have been issued in accordance with the provisions as they existed prior thereto had elapsed. In the present case, the period of six years from the end of the relevant assessment year 2016-17 had expired on 31.03.2024 and therefore, the notice issued under section 148 on 31.08.2024 for Assessment Year 2016-17 is barred by limitation and invalid in law. SLP of the revenue dismissed on account of delay of 309 days and also on merits. (AY. 2016-17)