Nudge start frame of a clip in the timeline?

I want to create a looping video with the least amount of difference where the loop goes back to the beginning.  I'm stacking the video twice i the timeline, then making the top one 50% opacity.  This works OK, but if I could nudge the positions with keystrokes it would be easier than trying to accurately move the clip by dragging with the mouse.

Does anyone have an idea?