Paid_until the free period of the subscription

I set up a monthly subscription with a free period of one month.
The paid_until was set 30 days after start_day, is this to be interpreted as an automatic update?

What do you mean by automatic update? The paid_until date would update once the subscription has been paid for. If the customer doesn’t pay for it, then the date would not change (even if the date has been passed).