Nice game! I know exactly what you're going for with the time-based farming, but think having an 8 minute timer before the zombies arrive is a bit too long for a game jam, when people spend an average of 5 minutes or less reviewing each game.
Also, a little music goes a long way! It's hard to squeeze audio in when we have a week to make a fully playable game, but even just grabbing a free 1-minute loop from somewhere will add a polished feel.
I got quite scared when the night came and suddenly there were zombies SPRINTING at me, which is exactly what I wanted. Well done there!