Left-click and drag to move a single marker to a new location on the timeline.
It is possible to move multiple markers by the same distance. Left-click each marker you want to move, then drag one of the selected markers to a new location. All selected markers will then move together. Note that the markers are bounded by the zero point on the timeline. In other words, the first marker in your selection cannot move to the left of zero on the timeline.
Both ends of a range marker
Left-drag either end of the range marker. The other end will move by the same distance.