A) from the TVideoGrabber SDK:

1) the OnFrameProgress2 and OnFrameBitmap events report FrameInfo->NTPFrameTime through the TFrameInfo structure passed as parameter of the event

2) invoke GetFrameInfo, e.g.:
__int64 NTPFrameTime = GetFrameInfo (0, fi_NTPFrameTime);

B) directly from the Datastead RTSP/RTMP/HTTP/ONVIF Source filter

– query the IDatasteadRTSPSourceConfig3 interface on the filter
– invoke:
__int64 NTPFrameTime;
if (DatasteadRTSPSourceConfig3.GetInt64 ("RTSP_VideoStream_Frame_NTP_time_100ns_int64", &NTPFrameTime) == 0) {
   // NTPFrameTime contains the NTP time of the video frame being rendered
}