The Sabbath is a religious observance, which no doubt is accurately kept track of. But whether or not the Sabbath is the seventh day depends on what calendar you're using. It's the seventh day insofar as the Hebrew calendar is concerned, but not necessarily for other calendars.But haven't the Jews kept the Sabbath as long as its been around, so wouldn't the day they keep today be the true Sabbath day ?.
The point is that the phrase "seventh day" is in and of itself is meaningless. It depends on what exactly you're counting.