You can define the loop points - aka the working range - of a Composition with the blue-ish mark on the top timeline, by dragging the left and right rectangle. When dragging that marker in between the left and right marks, the whole range will be moved.
When dragging the loops point, the points will be snapped to second. To avoid this behaviour hold down the Shift key while dragging.
By doing a right-click on the working range marker, you can Trim to Working Comp Area.
You can set the Working Area Start and End points to the current playhead from the Project menu.
Regions outside of the Working Area drawn with a black-ish overlay to help you recognising it easier.
