Interest for default in furnishing fringe benefits increases employer liability until the return is filed or assessment completed. Where a fringe benefits return is filed late or not filed, the employer is liable to pay simple interest from the day after the due date until filing or until assessment completion. Interest is charged on the tax determined on the value of fringe benefits, reduced by advance tax paid. The statutory definition of 'due date' applies and an assessment made for the first time under the specified assessment provision is treated as a regular assessment. Procedural provisions analogous to those for delayed income-tax returns apply as far as relevant.