I would like to track on my server when each user first subscribed. I hit the /subscriptions endpoint and check `subscriber.original_purchase_date`, but it is set to some date in 2013 (I only started implementing IAPs in the past couple of weeks). The property under `subscriptions.<product>.original_purchase_date` seems correct, but I would prefer not to have to check every product in that `subscriptions` dictionary for the earliest date, especially not knowing right now which products I will want to add later, and which of those I will count as subscriptions.
What is the purpose of `subscriber.original_purchase_date` and why is it set so early? Is it an artifact of Sandbox testing? A bug?