"It takes 24 hours."
Translation:Det tar ett dygn.
No, it's not the same as in French. Ett dygn is 24 hours, thus normally from 00.00 to 23.59 (but you could also say "jag har inte rökt på ett dygn" = it's been 24 hours since I had a cigarette).
It's quite funny that some languages have this (etmaal in Dutch, ööpäev in Estonian, vuorokausi in Finnish), and others don't. Or maybe some languages have had it, but it has fallen out of use?
Edit: Oh, and dag is just day like in English, normally from morning to evening, but it can also be used instead of dygn if it's not that relevant exactly how many hours we are talking about, for example if something happens three days from now.