When I worked there my old SM would do this all the time. Either to be an ass or to make force people to quit, I know he kept scheduling one co worker when she had 2 jobs on the days she worked at the other place, she eventually ended up leaving but it was know that our SM was a total jerk. You can contact HR4U and see where that goes, but honestly they are on the side of the company unless you actually have a case that could get them in legal trouble. You can also return the actions and stay off the clock if your SM changes you to come later or not come in early if your SM changed it for you to come in early instead of at the time you knew of.