Oh, so I already know what to do next in Thor
I think the client code of RenderWindow::GetFrameTime() becomes more complicated when game logic functions work with a
float dt (part of a second). However it would be inconsistent not to change it as the single function in SFML. Another option would is also to modify the logic ticks where possible and only work with milliseconds.
It's a good thing that you get rid of float's inaccuracy for long times. But aren't the floats theoretically more precise than a millisecond? As far as I know, QueryPerformanceCounter on Windows may have a higher resolution than 1 ms.
The addition of the 64 bit types is very nice!