Just because elevators and other things start at a 0 doesn't mean that makes sense anywhere else, it's fairly common for ground level to be 0. There is no instance where it makes sense to say 0 week = 7 days, 1 weeks = 14 days, etc. It does however make sense to say I live on the ground level (aka floor 0), I live on the 1st floor (aka second floor).
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3 weeks is pretty well known as 21 days.
Just because elevators and other things start at a 0 doesn't mean that makes sense anywhere else, it's fairly common for ground level to be 0. There is no instance where it makes sense to say 0 week = 7 days, 1 weeks = 14 days, etc. It does however make sense to say I live on the ground level (aka floor 0), I live on the 1st floor (aka second floor).