Damn, that derailed so quick.
It's kinda simple. Either it stays the way it is, has been and always will be or you just count the hours.
Make a NA, EU, CN Race and as a comparison just add two days for NA, one day for EU and zero for CN. Or subtract 24/48 hours.
If NA kills the Boss on the 2nd day of a new week and EU on the 1st day, EU wins. If both kill the endboss on the 2nd day after their reset and CN on resetday, CN wins.