The due date for the statutory audit is decided by the Governing body which prescribes the audit.
For example, as per Companies Act, 2013, every company is required to hold Annual General Meeting within 6 months from close of the Financial year and Company is required to present its audited financial statement in such AGM for adoption.
21 days clear notice is required to hold AGM. Accordingly, statutory audit under Companies Act should be carried out to ensure this compliance. Due date for the Tax audit for corporate is 30th September of the following Financial year. However, for FY 2019-20, it is extended to 31st October 2020.