How are speed run challenges timed?

Question

There are several achievements for completing each Act in less than an hour ("No Time to Rest", "Sandskimmer", "Swift Descent", "On the Wings of Angels", "In a Hurry?"). Can I do these runs on normal with a character who has already completed each Act? Do I need to do the runs with a character who has never been to the Act in that difficulty? Does the timer start when I enter the Act the first time, and does it mark an hour from that point, or only count one hour of gameplay time? How do these speed run timers work?

Answer 1

This is confirmed. I just completed Act 1 as a level 46 Demon Hunter who is half way through Nightmare and got the achieve.

As a sidenote, I believe this is about the only viable way to do these achieves. I have a run speed buff, and still only had about 3 minutes to spare despite running through mobs and not killing anything unless necessary.