There is no way to adjust your computer's clock back to the correct date without having some effects on the game. All of the timers in the real-time engine are affected by changes in the system clock, including hours played on the stats page. However, you can minimize the worst of those effects by taking certain precautions.
First, make sure that you have no nursing moms and that there are actually crops on the farm (it doesn't matter if it's full or not, just that there are some crops remaining), then pause each of the games when they meet those conditions.
Next, you'll want to back up your savegame files using the procedure in
this post. That way you can always go back to where you were if your games suffer major problems.
Finally, with the games still paused (and make sure that you don't have any other real-time games going on your computer that may also be affected, such as Fish Tycoon or VV1), set your computer's clock back to the correct date and time. If everything looks good in your games, you can continue to play from there. If not, you probably have no choice but to put the clock back to the wrong date, restore the games, and live with the problem.