If I wanted to shiny hunt smeargle in black and white in the swarms can I change my ds clock to the day it appears or what's the best way possible to hunt it
The gates in Unova have a monitor inside that will alert the player to where an outbreak is taking place. Unlike in previous games, mass outbreaks can be manipulated by soft resetting the game, but only if the player has not saved their game at all that day. When the game is restarted, there will be a different Pokémon swarming.
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The best way would be to check the monitor at gates in between routes and towns. Outbreaks change each day, don't occur on any specific day, and occur only after you enter the hall of fame and receive the national dex. If the swarm is for a Pokemon you don't want you are able to soft reset and the Pokemon that appears in a swarm will be different as long as you haven't saved on the day you check. If you do not leave the route or turn your DS off during a swarm, the swarm will never leave. Changing your DS clock doesn't work, because time based events will stop working for 24 hours.
Hope this helps.