Advance tax requires calculation during the financial year and payment after computing tax, credits, and applicable additions. Advance tax liability arises when computed advance tax meets the statutory threshold; resident senior citizens without business or professional income are exempt. Taxpayers under presumptive taxation must still compute and pay advance tax during the financial year. Computation aggregates income by heads, applies allowable deductions to obtain Net Income, calculates tax reduced by rebate, adds surcharge and cess, adjusts for specified reliefs, and subtracts prepaid tax credits to yield the outstanding advance tax liability.