Tax demand payment and default rules govern notice compliance, interest liability, instalments, and relief during appeal pendency. Tax demanded under a notice of demand is payable within the prescribed period from service of the notice, and the Assessing Officer may shorten that period with prior approval where delay is considered detrimental to revenue. If the demand is not paid within time, simple interest becomes chargeable on the unpaid amount, subject to reduction or refund where the tax liability is later reduced on account of rectification, appeal, revision, or settlement proceedings. On application made before expiry of the due date, the Assessing Officer may extend time for payment or permit instalments. Failure to pay within the original or extended time results in the assessee being deemed in default.