66. Advance tax shall be payable in four equal instalments on the fifteenth day of September, December, March and June of the financial year for which the tax is payable ¹[: Provided that, if before the fifteenth day of May of the year, an assessment of the assessee is completed in respect of an income year, later than that on the basis of which the tax was computed under section 65, the assessee shall pay in one instalment on the specified date or in equal instalments on the specified dates, if more than one falling after the date of the said assessment, the tax computed on the revised basis as reduced by the amount, if any, paid in accordance with the original computation.]