Move ONE whole caption row along the timeline by delta_ms (negative moves it earlier): its start, its end and every word timing move together, so the words keep their spacing. Pick the row by caption_id, or by at_ms: the visible caption showing at that timeline millisecond (hidden rows are skipped). The row stops at 0, at the timeline cap and at the visible caption on either side, so a move never runs into its neighbours. To change only a start or an end, use edit_caption. Refuses, naming the fix: neither caption_id nor at_ms given, an unknown caption_id, no visible caption at at_ms, or a delta_ms outside the range that fits (the refusal gives that range).