I touched the crystal in Anistar City, and then went to Victory Road and found the place where its supposed to be, right by the tower, changed my clock to, I tried both 20 and 21(I looked it up, it shouldn't be time locked since I changed the clock.) But what is happenning, why isn't it there?
I figured it out myself, so I'll post it here so if anyone's having the same problem, they'll know what to do.
My problem, was that I changed the time standing in the same screen as the Garchompite. Walking into the nearby cave and then back, made it appear.
I repeat: The Mega Stone will not appear, if it hits 8pm while you're on the same screen as it. Simply exit the screen and go back, and it will appear if this happens by accident.
Source: Just literally did it.