Get the shooting time of each frame from WebCam

Hi there!
I'm doing some WebCam programming and need to get the shooting time (system time) of each frame I got. I know in struct VIDEOHDR there is a member variable 'dwTimeCaptured' which can tell the elapsed time from first frame to this one. But how to get the shooting time of the first frame?
Thanks!

I hardly know much about webcams, but it's obvious your device must provide timecodes to recording facility. Otherwise you have to synthesize ones artificially on the recording side. Is this what you're asking about? :)