I'm not sure how it worked with the getting gold for one purchase promo, but mine has never been by calendar year. You can see in the app (and probably online somewhere) when your gold expires.
In history, mine says 'earn 0 stars by Oct. 31, 2017' to keep gold. Which means my gold is good until Oct. 31, 2018 (due to the promo, when you got gold with one purchase, those that already had gold got an extra year of gold for one purchase).
Never say never