How come biblical literalists are sure day means 24 hrs. but week means 6.8993839 years?

Most Christians these days read the word “day” in Genesis 1 and 2 as being allegorical, not literal. But a growing number of fundamentalists and evangelicals insist that they know it is meant literally, and that it tells them God spoke the Universe including our Solar System and Earth into being in 6 calendar days a little over 6,000 years ago. They are adamant in insisting that they know that day there in Genesis means a 24 hour day (one full revolution of the Earth, never mind that in Genesis it talks about a stationary Earth with the Sun revolving around it).

Yet when Daniel states a clear time from his writing till the birth of the Messiah in the book of Daniel 9:24–27, these same former literalists claim that the word week doesn’t mean 7 Earth days, but instead a period of 7 biblical years lasting 360 days each (or 6.8993839 years). This is pretty important because we know Jesus was born in 1 AD.

Daniel:24–27 says: “Seventy weeks have been decreed for your people and your holy city, to finish the transgression, to make an end of sin, to bring in everlasting righteousness, to seal up vision and prophecy, and to anoint the most holy place.

“So you are to know and discern that from the issuing of a decree to restore and rebuild Jerusalem until Messiah the Prince there will be seven weeks and sixty-two weeks; it will be built again, with plaza and moat, even in times of distress.

“Then after the sixty-two weeks the Messiah will be cut off and have nothing, and the people of the prince who is to come will destroy the city and the sanctuary. And its end will come with a flood; even to the end there will be war; desolations are determined.

“And he will make a firm covenant with the many for one week, but in the middle of the week he will put a stop to sacrifice and grain offering; and on the wing of abominations will come one who makes desolate, even until a complete destruction, one that is decreed, is poured out on the one who makes desolate.”

Daniel was writing circa 538 BC. Clearly, if 70 weeks means just 490 days, then Jesus was born over 536 years too late to be the Messiah. So suddenly, weeks must be taken metaphorically. But even the metaphors get messy. It requires a massive amount of hand waving to get any of Daniel’s prophecies to come out as pointing to 2013 current calendar years ago. If you wish to follow all the convolutions apologists like Josh McDowell use to do this, go to http://www.infidels.org/library/modern/jim_lippard/fabulous-prophecies.html#birth use your browsers page search feature to jump to “Daniel”. It’s really quite informative.

How do fundamentalists know when to shift between absolute, inerrant literalism and loosey-goosey allegorical interpretation of time?