Whenever you turn on Animal Crossing and load your game, Isabelle will greet you and help get you set up. Before entering town, you can ask Isabelle to change the current time and date, or to perform other tasks, such as demolishing your home, or even your entire town!
Be aware that changing the time and date can have dramatic effects on your game. For example, if you fast-forward too far, you may find that many of your neighbors have moved away because so much time will have passed and you weren’t there to convince them to stay. Think twice before messing with these powerful settings!