Not talking from experience, so I may be wrong, but it looks like your expiry date updates when you move up a level - so I imagine you’ll have platinum for 2 years from around the date where you qualify for it.

What’s making me think this is I signed up to RJ on 17th January, my status match to silver jay went through on 21st Jan, and my expiry date is now 21st Jan 2026, and no longer 17th Jan 2026.

I should hopefully be hitting Gold Hawk at the end of August, so hoping it works out like that.