12 Month Gold Subscription Not Lasting The Whole Year.

Did you have Game Pass Ultimate when you added the 12 month Gold?

Microsoft discontinued the 12-month subscriptions for Gold a couple of years ago. Anyone who did not cancel their subscription or upgrade to Ultimate, would be grandfathered into the 12-month subscription. Once cancelled, there is no way to go back to that.

The monthly price for Core will be the same as the currently monthly price for Gold. But again, as the 12 months subscriptions were terminated years ago, there is no equivalent price for that through Xbox.

You would have to start a new subscription to Gold

As Smutches said, 12 month subscriptions are no longer sold for Gold, so you will have to purchase 3 month subscriptions or find one of the remaining 12 month cards. You can stack up to 3 years of these, so buy 4 and you will have a full year.

What I see on that site is about 9 years and 2.5 months worth of Gold. It's a mixture of month to month, year subscriptions, and then 12-month prepaid cards, but my first Gold subscription started on 11/29/2003. I don't see any Silver subscriptions.

Since Ultimate includes Gold and Game Pass, if you already have those when you buy Ultimate, even use at trial, it cancels what you have and upgrades it to Ultimate as well. So your last 12 month subscription would have been cancelled when you used that trial and converted to Ultimate. In order to go back to just standard Gold, you'd have to either cancel your current Ultimate subscription or turn off auto-renewal and wait on it to end and then add new Gold subscription. However, Microsoft has ended 12 month subscriptions, so it would no longer be possible to purchase a $60 per year Gold subscription. You can still buy a 12 month prepaid code from a retailer.