If I have a PREMIUM account and I win on a trade that took two days to close, is the win divided by the number of days it took to close the trade, or is only the closing day taken into account?

A position that has lasted several days will only be taken into account for the day of liquidation and will only count for one trading day. If you close the position after 7 days, the day on which the position is closed will count as 1 day, not 7 days.