Advance tax obligation requires periodic self-assessment and payment; Assessing Officer can order payment and taxpayers may revise instalments. Every person liable to pay advance tax must pay, of their own accord, the prescribed percentages of advance tax on current income by the due dates, calculated under section 209, and may increase or reduce remaining instalments to reflect revised estimates. If already assessed for a prior year, the Assessing Officer may order payment of advance tax and issue a notice of demand, and may amend that order after a return or later assessment; taxpayers served with such orders may intimate lower estimates or must pay any higher estimated liability before the last instalment due date.