I'm going to be cutting it close for an SFO-ICN flight next week and was trying to figure out what time United will close the doors/gate.

Their website is pretty clear about a 15 minute cut-off for Domestic flights:
For flights within the U.S., all customers must be on board the aircraft 15 minutes before the scheduled departure time. We recommend being at the boarding gate at least 15 minutes before departure to ensure that you’re on the aircraft before the doors are closed.

However, for International flights it seems much more like a recommendation than a policy:
For international flights, we encourage you to be at the gate no later than 30 minutes before your scheduled departure.

Does anyone know what time they'll actually close the doors? I've searched pretty hard and come up empty.