Surcharge on income-tax increases liability where taxable income exceeds threshold; surcharge on advance-tax instalments must be paid by the due date. The Ordinance requires a surcharge to be added at a fixed percentage to income-tax computed for purposes such as deduction at source on salaries and advance-tax payments when taxable income exceeds the threshold, and it mandates that surcharge attributable to earlier advance-tax instalments be paid by a specified due date, with non-payment attracting defaulter consequences under the Income-tax law.