Hi Kinsi,
Subscriptions are not set in stone, which is why we highly advise you not to build your own twitchnotify type of extension based on exact renewal time guesses. They are highly affected by external payment provider services such as paypal, which callbacks can be hours, or potentially even a day late. In addition to this, the time differential for when a sub payment is authorized and processed, and when the chat message is displayed to clients may not be specific time accurate. We strive to make these as accurate as possible within rules that payment providers can allow for, but we are not able to say with 100% accuracy that all subs are processed exactly in 30 days.
Hope that helps.