The system will initiate a review of your Tier status on the last day of the Tier Validity Period (13 months). Our Dynamic Tier Review (DTR) system will review the InterMiles accrued against Qualifying Activities in your Account during the 12 months from the attainment of your Tier or for an additional 6 months period preceding the date of your Tier attainment i.e. for a total of 18 months. So long as you have accrued the required InterMiles under either of the 2 periods of evaluation (12 months / 18 months), your Tier will be retained for an additional 13 months