Great Game Overall
I think it's a great game, I've beta it 3 times. The only things it needs are weapon upgrades, a few more weapons, and a boss fight. I don't really know how a boss fight would be at the end, but it would be kind of dumb to just have a big freaking monster walk at you and you had to just shoot it before it reached your base.
I liked the Survivors concept, and the whole usage of hours thing. Also, make a Survivor Mode or something people can play continuously without stopping for the 20th day.
Great game. Especially for your first.