Calculate the real world time at which a frame was captured?

[George Hawkins]

If I use the filesrc element to playback a video file I can retrieve data
like this on a per frame basis:

    index=0, timestamp=832000000, stream-time=0
    index=1, timestamp=870000000, stream-time=38000000
    ...

But what is the first timestamp above relative to? How can I retrieve a
real-world start time such that I can combine it with this timestamp in
order to calculate the real-world time at which the frame was captured?

I control the original file capture process as well as the playback but I
haven't found how to capture and recover the start time that I need for
combining with timestamps in this way.

Currently, I capture the video file like so:

    gst-launch-1.0 nvarguscamerasrc \
        ! 'video/x-raw(memory:NVMM), width=3280, height=2464,
framerate=21/1' \
        ! nvjpegenc \
        ! matroskamux \
        ! filesink location=out.mkv

I can change the container and video format if this makes it easier to
encode and recover the start time later. I can obviously get an
_approximate_ start time by recording the time at which the pipeline
started - but I'd prefer something more precise (and _if possible_ I'd
prefer that the value was encoded somewhere in the resulting video file
rather than stored separately).

I've used GST_DEBUG to see if I could see anything that looked like a start
time when replaying the file but didn't spot anything.

And if I look at the file with a tool like mediainfo the only date I see is:

    Encoded date : UTC 2019-07-24 19:20:42

TL;DR - when recording my video file how do I capture and later recover a
value that can be combined with a relative timestamp value (like the one
for index 0 up above) to give the real world time at which the frame was
captured.

For reference: I retrieved the above timestamp values etc. from the command
line like so:

    $ GST_DEBUG=GST_BUS:5 gst-launch-1.0 filesrc location=out.mkv \
        ! matroskademux \
        ! multifilesink post-messages=true location=/dev/null &>
gst-bus-debug.log

    $ sed -n 's/.*gst_bus_source_dispatch.*, \(index=.*\)/\1/p'
gst-bus-debug.log

Regards,

/George
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