How long is a Biblical Day?

In case you're a literalist wrt Gen. 1, here's one wacky possibility:

Note that Venus has one face always facing the sun. Perhaps in the past Earth was like Venus, and had only one face towards the sun. The rotation it has acquired today may have come only gradually, so in fact the saying may hold true, "A thousand years is like unto a day" (perhaps even a billion...?).