Each subscription has a renewal date computed from its last renewal anchor (Previous Billed Date's underlying plan-renew date) plus the plan length in weeks, moved to the subscription's Day To Renew weekday. If the plan has Buffer Days to Charge Before Renewal, the charge can happen up to that many days earlier – the subscription page shows this as Next Billing Period (buffer date – renewal date) when a buffer applies, or Next Billing Date when it doesn't. An automated routine (run on a schedule set up on the server – commonly daily) charges every active subscription for which today falls inside that window, or whose renewal date has already passed and which is not flagged with an error. The Renew column on the subscriptions list is highlighted while inside the window and flagged once the date has passed without a charge. Paused, cancelled and suspended-customer subscriptions are never picked up.