If you stop changing the clock on your device and let time progress normally, it's possible that the game will eventually correct itself, but it's not guaranteed, depending upon how messed up it is.

If you delete and reinstall the game, you will have to start the game over from the beginning, although you won't have to buy it again. Apple has some very good help for the App Store and fixing problems. Just go to this page on their site, scroll down and click on "App Store" under Customer Service, then select Troubleshooting Applications to see the article on Redownloading purchased apps from the App Store.