The hack must be built during MHacks 12 and within the 36 hours of hacking.
If you're a beginning hacker, check this out: http://blog.mlh.io/your-first-hackathon-10-07-2013/
In addition to Major League Hacking's Code of Conduct found here we ask hackers at MHacks to agree to the following (some content sourced from http://hackathon.launch.co/rules):
1) Treat all other hackers with utmost respect. Be kind to others. Do not insult or put down other attendees. Behave professionally. Remember that harassment and racist, sexist, or exclusionary jokes are not appropriate for this event. If at any point you see a fellow hacker being harassed, it’s your responsibility to talk to the nearest hackathon organizer.
2) Treat our sponsors with the utmost respect. Without them nothing would be possible. Take some time out of your work to go meet and speak with them. If they come over to talk to you look up from your work and give them a bit of your time. They're here for you! Show them you appreciate it.
3) The hackathon is a walled garden. To ensure a level field for all contestants, all code, design, art, music, SFX, and assets must be created during the duration of the hackathon. We want to ensure that all participants start off on the same footing and we also want to preserve the true nature of a hackathon. You are, however, free to make plans, create wireframes, and brainstorm prior to the event.
The only exception to this rule is that you may use legally licensed material and tools that are freely available to the public. Some examples of these would be: public domain images, creative commons music, open source libraries, and existing APIs and platforms. Failure to comply may result in the offending team's disqualification.
TL;DR: Bring your blueprints, build at the hackathon. It’s the only way we can compare hacks on a level playing field and fairly award prizes. It’s what lets you say “I built this at a hackathon”.
4) Have fun. Hackathons are amazing, and so are you. We’re so happy you’re able to hack with us and be a part of our amazing community.
5) Open your mind. Hacking unites people from across the world and from different cultural norms, nationalities, and backgrounds. Be prepared not only to learn something new from your hack, but also from the amazing people around you. Be mindful of the fact that certain content and actions can make the people around you uncomfortable. If your hack contains material that might cross that boundary, talk to an MHacks team member. They'll review it with the team and notify you if you should go forward. Don't waste 36 hours on a hack that won't be allowed to demo.
6) Be the change you want to see in your local community. Never be afraid of competing based on where you come from or have preconceptions of grandeur because you come from a great school. One can achieve greatness at any time. You simply have to see it and grab it.