The Bible is its own best interpreter. As much as possible, other portions of the Bible – and particularly surrounding portions – should interpret the meaning of any word that is ambiguous.
Genesis Chapters 1 and 2
It's all about 'yom'
The Hebrew word for 'day' in the Genesis account is 'yom,' which like the english word 'day,' usually means a 24 hour period but can also mean a long indefinite period, like we say this is the day of social media.
GENESIS 1: 11-12. At beginning of Day 3, God said, 'Let the land produce vegetation: seed-bearing plants and trees on the land that bear fruit with seed in it according to their kinds. And it was so. The land produced vegetation: plants bearing seed according to their kinds and trees bearing fruit with seed in it according to their kinds. And god saw that it was good. And there was evening, and there was morning – the third day.'
It takes more than 24 hours for land to produce and reproduce vegetation and bear fruit.
GENESIS 1: 5, 8, 13, 19, 23 and 31. The phrase 'And there was evening, and there was morning – the [number] day' is the closing statement for each of the six days of creation but not for the seventh day of rest (pause in new creations).
Therefore, the seventh day is still open, and since the closing statement is not used, the word day obviously means more than 24 hours because more than 24 hours has passed.
GENESIS 1: 5, 8, 13, 19, 23 and 31. The phrase 'And there was evening, and there was morning – the [number] day' is the closing statement for Days 1, 2 and 3. But the sun didn't come until Day 4, for purpose of 'signs and seasons, and for days and years.'
The concept of a 24-hour day came AFTER yom was already in use to specify beginning of something and end of something.
GENESIS 2.4*. This verse says, 'This is the account of the heavens and the earth when they were created, in the day the Lord God made earth and heaven.' The previous chapter says God made earth and heaven in six separate days.
Therefore, this use of the word day (singular) is more than 24 hours because it is the sum of six days (plural).
GENESIS 2:16*. God said Adam must not eat the fruit from a particular tree, 'for in the day you eat from it you will surely die.' He ate the fruit but did not die within 24 hours.
Therefore the use of the word day here must mean a future period of time.
GENESIS 2:18-20. This passage says that God gave Adam the assignment of naming every living creature ... that 'It is not good that the man should be alone' ... and that 'there was not found a a helper as his partner.'
It is unlikely that completion of this assignment and Adam's loneliness would all occur within the first 24 hours of his life. The word day here seems to mean a longer period of time.
Other than the way 'yom' in Hebrew was translated 'day' in English – a translation problem – there is nothing in these verses that even suggests that each of God's creative processes was limited to 24 hours.
* The New International Version (NIV) is the only major translation that, erroneously and for no apparent reason, omits the word day in these two passages as it is written in the original Hebrew text.