updated_at in the user object is the timestamp of when the user last updated their profile.
I have been looking at similar recently.
Basically you have to do the math yourself.
But looking at my PayPal Twitch renews the sub after the number of days that are in that month. So you can math from there.
Currently theres no notification or easy way to spot a automatic resub.
The other thing to consider is that the resubs tend to go thru in the early hours of UTC, (anything between 0am and 8am from my PayPal receipts). So theres no real worth to you doing a in chat notification if that is your aim. Because they’ll either all come in at once, or all fire when your caster is offline.