Late is more important than early with Jap millet. You'll want to plant next year around the 3rd week in July or 1st of August. Time it where the ground is dry when you plant, and then a good hard rain a couple days later. Spraying a pre-emergent to limit
Johnson grass and weeds beforehand is a good idea. Once millet is a couple inches high it doesn't matter if it rains a bunch, but if it rains too much when it is trying to get up, it will rot in the ground. Also, if it doesn't get enough rain right after planting it will not germanate. It is tricky, but man when you get it right!!! I usually get hair from the barber shop and put it up all around to keep the deer out, or limit how much they eat when it is growing. Once the seed stalks form deer will not eat it anymore.
Cover is probably the most overlooked part about building an impoundment or attracting ducks to an existing area. Ducks have a "morning feeding area," a "day roost," an "evening feeding area," and a "night roost". You want that day roost and night roost undisturbed. You also want standing timber and bush around and in an impoundment. In one of my impoundments we planted cyprus trees and water oaks which are both about 20ft tall now. The idea is to make it look "ducky" and not just an open watershed like a pond. Last year was the first year out of 6 that ducks really used that impoundment. There were several hundred woodies, mallards, ringers, scaup, shovelers, and teal using that 3 acre hole. It was planted in corn, with the corn left standing, and millet around the edges. We never hunted it. Why, well, we mave have killed some ducks there, but about 20% of our ducks are return birds from previous years. They will remember getting shot at in a "new" place alot more than one that ducks have used for years. It takes time for an impoundment or duck pond to "mature" just like it does for a deer. Our oldest impoundment is 15 years old this season, and is 12 acres of flooded corn. Last year you could have taken 3 guys in there every day of the season and limited on woodies, mallards, blacks, and ringers in about 3 minutes. We hunted it twice a week and every hunt but one was a limit for each gun. It is just an old field in the middle of nowhere that we dyked and put a water control gate in and then ran a pipe down to the swamp. If I showed you that place in May you would laugh and say I was crazy to think I could kill ducks there.
Anyway, good luck and good hunting.
Oh, put up some wood duck boxes. It is alot of fun to see the ducks use them. We have about 100 up now, give or take. We have about 90% nest rate and about 70% hatch rate. Figure an average of 8 birds per box and that's about 800 birds per year that are semi-resident.