One of the first 4Chan threads I could find started at 9:15 Eastern on 3/14/19. The screenshot posted (assuming it was taken immediately before posting) says the video was streamed live "39 minutes ago", i.e. at approximately 8:36 Eastern. In NZ time, that means the 4Chan post was made at 2:15 pm, with the broadcast starting no later than 1:36 pm. He fires his first shot about 6:30 into the video, so that means if the video was broadcast live, he fired the first shot no later than 1:42 pm. Since the timeline has him firing the first shot at 1:40, either they're 2 minutes off, or it took the 4Chan poster around 2 minutes to make the post after making the screenshot, which is reasonable.