Export profit deduction timing limits qualifying period for claiming income-tax export benefit, changing eligibility window for taxpayers. The amendment to section 80HHA inserts a temporal limitation into sub-section (2), clause (i), so that the reference to the 30th day of September, 1977 is qualified by the words 'but before the 1st day of April, 1990,' thereby establishing an explicit cut-off date for claiming the export-related deduction.