I connected a daylight detector, through a NOT gate, to a sticky piston with an attached stone block in front on a villager's door. Before sunset, the villagers head inside, and then afterwards the daylight detector pushes the stone block in front of the door; effectively protecting the door from zombie attack during the night. In the morning, the daylight detector removes the stone block from in front of the door, allowing the zombies to attack it.

Zombies should stop attacking doors at the same time, or slightly before the time, that daylight detectors determine it's day.