The primary representation in the current software is frames, according to the
time scale (frame rate) indicated. The time scale is not the FINAL frame rate
of playback, it only represents the time scale used to convert between time
and frames when deciding what time to play content and when.

Our software since the beginning used to represent software in seconds, but
there were some floating point precision issues with it that we fixed around
2010 or so by adding the frame numbers. The seconds fields are still there to
this day for backwards compatibility.

