Such disclaimer shall be filed at any time after the creation of the interest, but in all events within nine (9) months after the effective date of the nontestamentary instrument creating the interest, or, if the disclaimant is not then finally ascertained as a beneficiary or his interest has not then become indefeasibly fixed both in quality and in quantity, such disclaimer shall be filed not later than nine (9) months after the event which would cause him so to become finally ascertained and his interest to become indefeasibly fixed both in quality and quantity. With respect to a disclaimer, an interest which is a joint tenancy interest in property shall not be considered to be indefeasibly fixed both in quality and quantity until the death of all but one of the persons owning such joint tenancy interest.