Points will expire based on a rolling 180-day basis. If you do not use your points within 180 days of earning them, you will lose them.
Tier status is based on your spend within the last 365 days. You will have 365 days from the date you join a tier to reach the next one. The tier you are in after 365 days will be the tier you are in for the next 365 days. To remain in that tier, you must re-qualify by spending the dollar amount threshold for that tier, or you will move down tiers.