update begin/end time of children only if parent unmodified
update begin/end time of children only if parent unmodified


ebu/ebu-tt-live-toolkit,bbc/ebu-tt-live-toolkit,bbc/ebu-tt-live-toolkit,bbc/ebu-tt-live-toolkit,ebu/ebu-tt-live-toolkit,ebu/ebu-tt-live-toolkit