Why is it that in French, "huit jours" , which literally means eight days, can also mean "a week"?

We were discussing this in French class, and we couldn't come up with a logical answer. Thanks!
Update: It was made as to be an equality in our French book. For example the book says "Il est resté huit jours." is equal to "He stayed a week." When I searched the internet, I also saw "après huit jours" meant "after a week". Not sure which francophone country or countries this... show more It was made as to be an equality in our French book. For example the book says "Il est resté huit jours." is equal to "He stayed a week." When I searched the internet, I also saw "après huit jours" meant "after a week". Not sure which francophone country or countries this may be used in, if any.
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