Put short, changing the clock / date on your DS does NOT make getting any of those Pokémon come sooner. I know, I've tried.
You can only get Shelgon after 70 real-life calendar days go by. Be sure to keep tabs on how long each section of the Safari Zone has been "active" on your calender. So go back to the Safari Zone on May 9th 2013 to try to catch a Shelgon.