Regardless of how many Tier Miles you have earned or flights you have flown, your card validity and benefits are extended beyond the tier review date by an extra two months, even if you have not qualified to stay in your tier.
For example, if you achieve Silver membership on 17th June 2016, your tier review date will be at the end of June 2017, but you can still use your Silver card to enjoy extended benefits until the end of August 2017, even if you do not retain your tier.
Tier reviews always take place at the end of a month.
Learn about how to keep your tier status.