Advance tax obligation requires taxpayers to self-assess and pay instalments based on estimated current income by due dates. A person liable for advance tax must on their own initiative pay advance tax instalments calculated from the specified sum, defined as the assessee's estimate of current income, at the appropriate percentage by each instalment due date. A taxpayer who pays earlier instalments may increase or reduce remaining instalments so total payments accord with the specified sum and the advance tax payable thereon.