(b) the 15th day of September, the 15th day of December and the 15th day of March, in any other case. Provided that where, in respect of any class of assessees, the Board has, in exercise of the powers conferred by the proviso to sub- section (1) of section 211 of the Income- tax Act, authorised the payment of the last instalment of advance tax on the 15th day of March during the financial year instead of on the 15th day of December, the last instalment of advance surtax in the case of such assessees shall also be payable on the 15th day of March during the financial year.