The Unix time in microseconds mx plays against: the system's, or while a song renders offline, the time its render has reached.
A render sets the time with start_render/1 and moves it on with advance/1, so the band, its loops and the engine work out a song as they do when it plays, only as fast as the render goes.
Summary
Functions
Moves a render's time on to at_us.
The time now, in Unix microseconds.
Whether a song is rendering offline, so the time moves only as the render moves it.
Stops following the system's time and holds the time at start_us until advance/1 moves it.
Follows the system's time again.