Pokémon ultra-sun and ultra-moon has been out for almost 2 weeks I restarted the game, and I want my trainer I'd to say I started my adventure on the 18/11/17 the day after release, it is the 28th of November in Australia, a whole 10 days after release. If I change the time back to the 18th of November 2017 play for a bit and save, then change it back to the current date whole it affect the game play itself? Would they be counted as legit not hacked or glitched Pokémon? And would I be able to go to Pokémon competitions with the same game?
No, the only thing that will happen after you change the date will be not being able to do time-based events for a day as already stated in the comments. Your game will not be counted as hacked since you didn't change the games code itself, and you will still be alowed to go to competitions.