The group life insurance policy shall contain a provision that the policyholder is entitled to a grace period of not less than 30 days for the payment of any premium due except the first, during which grace period the death benefit coverage shall continue in force unless the policyholder shall have given the insurer written notice of discontinuance in advance of the date of discontinuance and in accordance with the terms of the policy. The policy may provide that the policyholder shall be liable to the insurer for the payment of a pro rata premium for the time the policy was in force during such grace period.