Why does the word in Genesis for 'day' mean a 24 hour day, whereas the word for day in Day of the Lord does not?
Why not use the word which means 'it could be a millenium' day instead of the 'morning and the evening were the first (24 hour) day?
I'm reading in an NIV version in Genesis 1, and it has phrases like: "And there was evening, and there was morning-the second day."
It is possible to assume this is talking about a sideriel day - and I believe a lot of folks draw that conclusion. It is not impossible, again, to interpret this as being a day in the life of God - which, of course, spans all time.
Beyond that, I'm drawing a blank of a distinct reference from the Book that identifies the length of day as a sidereal day.