You could use the API to pull the subscription [created_at] date time.
https://api.twitch.tv/kraken/channels/:CHANNEL/subscriptions/:USER
This requires an OAuth with permission from the streamer for the scope [channel_subscriptions]
Or the or from each user using the scope [user_subscriptions] (probably not preferred for obvious reasons)
Changes to payment methods and some gaps in renewal times reset this field, whereas the IRCv3 tags would continue the subscription uninterrupted.