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36 [ 31 Time for payment of contribution. -An employer who is liable to pay contributions in respect of any employee shall pay these contributions within 21 days of the last day of the calen­dar month in which the contributions fall due:] [Provided that where a factory/establishment is permanently closed, the employer shall pay contribution on the last day of its closure:] [Provided that an employer may opt, in such manner as may be prescribed, by the Director-General for payment of amount in advance towards contribution to be adjusted against contributions payable by him (including employees' contribution) for a wage period so that the balance of advance amount continues to be more than the contributions due and payable at the end of the con­cerned wage period. Such an employer shall furnish in the pre­scribed proforma [(Form 5A)], a six monthly statement of contribu­tions payable and paid in advance with the balance left at the end of each month along with return of contributions to the appropriate Regional Office of the Corporation.]

37 [ 31A Interest on contribution due, but not paid in time. -An employer who fails to pay contribution within the periods speci­fied in regulation 31, shall be liable to pay [simple interest at the rate of [twelve per cent.] per annum] in respect of each day of default or delay in payment of contribution.]