[xiamenese]

As a kludge, could you create a small shape, with no stroke and perhaps no fill, but with a magnet at its centre. Position this vertically below the box where you want it, draw the arrow down to that shape then group the whole. Then you can move it around as you like and the arrow will remain in the same apparent orientation to the box, and you can place the shape wherever you want it to be in relation to the timeline.

Any good?