Regading tax regime

A user filed their income tax return under the old tax regime with no demand or refund. They later filed a revised return under the new tax regime but received an intimation calculating tax under the old regime, resulting in a demand notice. A Chartered Accountant explained that the option for the new tax regime must be exercised before the due date for filing the ITR (Section 139(1)). Since this wasn't done, the tax calculation under the old regime by the CPC is considered correct.