LOL no no, Time Travel is a trick where you set your DS internal clock to change the game's clock to your advantage. In other words you can cheat to get the item you've missed in the shop a few days earlier. Or set your DS clock to your birthday to get your birthday present... Its not recommended.
Does it ever cause problems in online multiplayer? I kinda always assumed it did because it's a category for the spreadsheet.
Y'know, I'm pretty sure all those years back I time travelled on the DS version multiple years into the future to see what would happen. I got weeds everywhere, and the little me was so upset.