From what my father told me, feel free to correct me if what I'm quoting is wrong, the term "days" is simply a mistranslation, and is intended to mean "a period of time"
God created the universe in about 7 "periods" from what he told me.
It's a case of personal interpretation. There's no point arguing over mistranslations, since ultimately the Bible has been translated thousands of times. It's a massive game of chinese whispers, and for some time of it's history it's not even been written down, just preserved orally.
The two main interpretations are those of Young Earth Creationists and Old Earth Creationists.
Young Earth Creationists come to the conclusion that the earth took 6 literal days, of 24 hours, to be created.
Also that the Earth is only 6000~ years old, and this is evidenced in the Bible by the ages of the Biblical Patriarchs, the first men, who lived for hundreds of years and even millennia. Add up all the ages between births in the Old Testament and the world is about 6000 years old, as first worked out by
Archbishop James UssherOld Earth Creationists tend to find that the Bible is mostly metaphorical/allegory, and that it's not the precise word of truth, but is instead inspired by the teachings of God/Jesus, and to a lesser extent is inspired by the cultures of the societies who wrote it.
They believe that the Earth/Universe was indeed created in 6 'days', but these days do not exist of 24 hour periods.
Instead they stretch for hundreds of thousands, or millions, or billions of years. Each day isn't necessarily the same length as the others, but each are definitely longer than 24 hours.
To them the word 'days' can be better read as 'ages'. So, the world was created in 6 ages, which spanned billions of years.
The distinction between the two is often that the Young Earth Creationists tend to be fundamental christians/muslims/jews, and they believe that the precise word of the Bible/Quran/Torah is literal truth.
In a lot of countries, particularly the west (bar the USA) there are fewer fundamentalists, and instead most believers take their holy books not as divine law, but as an inspired book of events to teach through metaphor and allegory.
Here's an interesting (albeit not particularly unbiased) video documentary in which several British fundamentalists are taken on a roadtrip to challenge their fundamentalist views. It's not strictly an anti-religion programme, but it has a bit of that undertone. The intent was more to make people think of religious books as more like metaphor than written law.
Just note, some people in this video are proper richardheads (the host included at times).
https://www.youtube.com/watch?v=Oju_lpqa6Ug 
