I'm not 100% sure of the date you initially set your tier status renewal period, but I feel 99% sure it's from the date you first accumulate 400 points to reach Silver. That first milestone would occur within a rolling 12 month window with no set dates: it will be rolling from the date of the oldest flight within the prior 12 months, I believe.

However, as long as you've earned any tier points in the last 12 months, then I think when you login to MyAccount, it will tell you the date by which you need to earn 400 points to get to Silver. I just looked at my son's account (he has zero tier points but he did have 60 tier points from a flight over a year ago) and there is no date given for him to reach silver. It just says "Earn 400 more Tier Points to level up to Silver Tier membershipand unlockmore excitingrewards."

I'm 100% sure that thereafter, the renewal date never changes. So, how ever many tier points you earn by that date will determine your status for the the following year.