It sounds like the seek method would do the trick.
Example
var start_position_seconds = 0.0
var animation = get_node("AnimationPlayer")
animation.play("your_animation")
animation.seek(start_position_seconds, true)
Do note, that in the example above, we set "true" in animation.seek(startpositionseconds, true). This is because our example here is an animation that we only run once and thus need to restart it.