Carryforward of specified business losses limited to profits of similar businesses; timely return filing required to preserve carryforward. Losses of a specified business under Section 35AD may be carried forward indefinitely and set off only against profits of other specified businesses; the loss is eligible for carryforward and future set off only if the return declaring the loss is filed within the due date prescribed under section 139(1), and a return filed after that due date disqualifies the loss from carryforward.