Where does total time in OSD come from?

[giraffedata at gmail.com (Bryan Henderson)]

In the OSD status display, it shows how far into the file I am and the
total running time of the file.  Where does the total running time
come from?

I'm talking about an MPEG file and ffmpeg codec.  The MPEG file was
created by IVTV.

What makes me the most curious that occasionally, the total time
displayed is less than the real running time.  So e.g. near the end of
the file, the OSD might display "00:40:00/00:26:00".  Also, I know
IVTV has no way to know as it creates the stream when the stream will
end, so I know that information isn't explicit in the file.