CA11570 - General: Definitions: Normal time limit for amending an income tax return
CAA01/S577 (1)
The normal time limit for amending a return for the year ending on 5 April is the first anniversary of the 31 January following that year. For example, the time limit for amending a return for 2016/17 is 31 January 2019 because 2016/17 ends of 5 April 2017 and so the 31 January following the year 2016/17 is 31 January 2018 and its first anniversary is 31 January 2019.