GH Classic: Will the real Shabbos please stand up ?

Something that has bothered me since I was younger. Clearly, even according a haredi history, some funny things have happened to our calender over the millenia. This being the case, how do we know Shabbos is really Shabbos ? Maybe its actually Wednesday, as josh narins mentioned in a comment.

Alternatively, if the universe really is 14.5 billion years old (give or take a few billion), when is Shabbos exactly ?

I guess the answer might be that it doesn't matter which day it is, as long as its one out of every seven. Or alternatively, Hashem has always worked behind the scenes to ensure that Shabbos is actually held on Shabbos.

Also, once I realized that the universe is quite old, kiddush friday night started to bother me. But then I realized, our havanah in vayechulu is pretty basic anyway. I mean, what does "Shovas " mean. Hashem is tired after a long week of universe construction ?