the idea of a game jam is to create a game in a limited amount of time (and two weeks is pretty generous). it's scary and exciting with the deadline but it's encouraging because many other people are working in the same time. especially with a jam so focused on learning new things, experimenting, and taking first steps, we have quite a few jammers who try stuff out a few days before the jam.
we encourage you to work within the time confines of the jam (making the game in two weeks isn't the focus, it's making it in the two weeks with everyone else that's cool), but we won't ban a project because you might have tested a thing or two beforehand. this is especially if you're away from a computer or workspace during the jam timeline.