If you are looking for information regarding the badge they will have in chat, the best way is via the tags sent alongside the message
ex. @badges=moderator/1,subscriber/6,premium/1 – subscriber/6 indicates I have the 6 month sub badge, and when I hit 12 months it’ll update to /12 but not before then.
Beyond that, you can use the ‘created_at’ date (on the subscription level, not the user level) but I am 99% sure that data resets if the user doesn’t renew their subscription immediately but still re-subscribes within 30 days. They keep their streak but the returned date in the api is incorrect.
So, there isn’t really a 100% perfect way to identify when a user had started their subscription (as they can let it lapse but then sub again within 30 days to keep streak).
If a user doesnt let their subscription lapse, then their object will be returned the same month over month.