If I buy a non edible item on the 1st of Safar, and my date to pay khums is 1st of Rabi ul Awwal. The item is not used till this particular khums day do I have to pay khums on it or the Khums will be payable on that particular item if I don't use it for 1 whole year (ie The non edible item is not used for one year from 1st of Safar)?

Yes. You have to calculate it among the income of the elapsed year.