Any distance between two things is called a span. These end points can be physical, like the span of a rope between two trees, or they can be more abstract, such as the span of time between five and six o'clock.
Span comes from the Old English spann, the width of the stretched-out hand, measured from thumb to little finger. It came to refer to various other measurements, such as the distance across an arch. The first reference to a length of time came in the late 16th century, and the phrase "attention span" first appeared in 1922 (as "span of attention").