Deemed default for unpaid advance tax arises when instalments aren't paid or required estimates/intimations are not furnished. Failure to pay scheduled advance tax instalments or to send the required intimation or to pay on the basis of an estimate of current income by the specified date results in the assessee being deemed to be in default in respect of such instalment(s); a lawful deferment of payment postpones deeming of default until after the communicated deferment date.