Section 153(1)

(1) No order of assessment shall be made under section 143 or section 144 at any time after the expiry of twenty -one months from the end of the assessment year in which the income was first assessable: 2[Provided that in respect of an order of assessment relating to the assessment year commencing on the 1st day of April, 2018, the prov isions of this sub -section shall have effect, as if for the words “twenty - one months”, the words “eighteen months” had been substituted: Provided further that in respect of an order of assessment relating to the assessment year commencing on or after the 1 st day of April, 2019, the provisions of this sub -section shall have effect, as if for the words “twenty -one months”, the words “twelve months” had been substituted. ]