From a lecture by Dr Joseph Pipa on the meaning of "yom" in Genesis 1?
"...it is used 2304 times in the Old Testament with the great overwhelming majority of its uses referring either to the period of daylight or the normal 24 hour day. The theologian Berkhoff writes "in its primary meaning the word "yom" denotes a natural day".
It is a good rule in exegesis not to depart from the primary meaning unless this is required by the context. It is not only the primary meaning, it is the default meaning of the word... this is the overwhelming sense of the word and that your first reading is always to be then of a daylight period or a normal 24 hour day, unless there is something else in the text that will cause you to see that it is being used in a figurative fashion."
Critics will ask how can you have a standard day in days 1 through 3 without the sun to regulate the period? But clearly days 4-7 are regulated by sun and should be so taken, so the coherent reading of the text, if 4-6 are regulated by the sun, then the same meaning should apply to verses 1-3: that's what the text demands.
Another word in the Hebrew that could have been used to communicate long periods of time is "olam". 'Day' does not express an epoch, if one claims then, one has to demonstrate such a use in the Bible: is there any such use of day in the Bible? Even figurative uses rely on day indicating a 24 period, except where is it idiomatic. For instance "period of influence", when it still contemplates a set of natural days or 'when'.
576 times "yom" with a numbered prefix: always refers to a standard day; the only possible exception is Hosea 6:2, but here, still, it refers to not a long period, but of God's brief judgement. It is also a prophesy of the resurrection, and still refers basically to a day as a day.
Moreover, when an ordinal is used with "yom" it always means normal day in sequence.
[Pipa suggests that you check the usage of 'yom' in Bibleworks or Logos...my preference is for Bibleworks, FWIW]
