Something like https://commons.wikimedia.org/wiki/File:Ljubljana_Time-lapse_-_Castle_view,_New_Square,_Ljubljanica.webm?start=0:30 should jump to 0:30 when the video starts playing.
Embedded files already support the start=mm:ss and start=sss formats, e.g. [[File:foo.webm|start=0:30]]