Tiers are based on 12 months of rolling spend less any discounts or gift cards. If you do not spend the minimum amount required for that tier, you will moved down to the appropriate tier. Points expire 12 months from the date of issuance. If you have not earned enough points to stay in the same tier, you will lose your tier status when those points expired. But the good news is that you can earn bonus points again for re-entering that tier.