This should help you measure correct time values: https://irlans.wordpress.com/2015/01/16/asynchronous-keyboard-input/

Thank you for posting that, I was wondering how this would work with inputs.

There is one more question I have, and that is about what timestep I should use. I was wondering before if the timestep would be equivalent to the best fps my game would show, e.g. 16 = 60fps. I am doing interpolation as well, so how smooth the movement is should be dependent on if I can get a new state to interpolate with every cycle of the loop I think? Or does the interpolation automatically smoothen the movement since my accumulator still gets higher so every frame drawn will still be different?

Obviously if a computer can't reach 60 FPS then each frame more steps are going to be taken and the rendering is going to be slow. So basically for the first statement: no, it varies between computers.

I don't know if I understand your second question. AFAIK linear interpolation as in here smoothes rendering and not game updates (A.K.A ticks). I've never used to interpolate between body dynamics. Sorry!