Edge case question for the brains trust here ...

I'm currently silver with a renew date of 5 Jun. My profile says I need 22000 + 4 before then to upgrade to gold.

In May, I'll fly 18000 miles + 4 sectors reqaulling me to silver. In July, I'll fly another 12000 miles.

Is the upgrade window a rolling 12 months (so I'll be gold in July) or fixed based on my silver date (so I requal silver in May, tier miles expire on 5 Jun, and requal silver again in July)?