What is the “tier anniversary date”?

Your tier anniversary date is the date you joined the current Club Cage loyalty program. It’s from this date that we start counting the points you earn over a 12-month period.  
At this date every year, your tier points will be reset, and you will then have a new 12-month period to accumulate points and reach a new tier level.  
To learn more about this, read this article. 
You can see your tier anniversary date in the mobile app by clicking on the button More and then on Club Cage Tier Levels.