Extended Timelines

I would like to be able to retime my keyframes so that they start before 0:00, similar to the way After Effects allows you to drag the placement of a layer prior to the first frame of the timeline. This would allow animators to build manual loops that begin at 0:00, but control which point of the loop is at the start of the timeline. Ideally the time prior to 0:00 would be visible in the timeline editor.