Advance tax instalment schedule requires staged payments by quarterly deadlines; presumptive taxpayers must pay by the final annual deadline. Self-assessment tax must be paid before filing the return of income, with late payment attracting interest under section 234A. Advance tax for regular taxpayers is payable in staged instalments across the financial year, culminating in full payment by the final prescribed date, whereas assessees under presumptive taxation (sections 44AD and 44ADA) must pay advance tax by the single last prescribed date of the year.