Defining "timeline offset" for various formats

Hi,

I was recently trying to implement
HTMLMediaElement.getStartDate()<http://www.w3.org/html/wg/drafts/html/master/embedded-content.html#dom-media-getstartdate>in
Blink & Chromium and found myself trying to figure out what the
"timeline
offset<http://www.w3.org/html/wg/drafts/html/master/embedded-content.html#timeline-offset>"
should be for different formats. For now I only have WebM implemented, but
I thought it might be useful if we all agreed on what this value should be
for Ogg and ISOBMFF as well. It seems like this could become part of
the HTMLSourcingInbandTracks spec.

For WebM/Matroska at least the
DateUTC<http://www.matroska.org/technical/specs/index.html#DateUTC>element
maps to the "timeline offset" concept, but for now I only expose it
if Duration is not set and the Segment has an unknown size. This is because
"timeline offset" seems to only be useful in a live context and no duration
& unknown segment size is the primary way to detect a live stream in
WebM/Matroska. I'm assuming that similar hoops have to be jumped through
for other formats and I think it would be nice to write those down so
everyone implements them the same way.


Aaron