Various models of time have been proposed in the philosophical and logical literature of time. These view time, among other things, as discrete, dense, or continuous. Intuitively, the instants in a discrete model of time are isomorphic to the natural numbers, i.e., there is the notion that every instant has a unique successor. Instants in the dense model of time are isomorphic to (either) the real or rational numbers: between any two instants there is always another. Continuous models of time are isomorphic to the real numbers, i.e., both dense and also, unlike the rational numbers, with no ``gaps.''
In a data model that supports a time line using chronons (isomorphic to the natural numbers or a subset thereof), an instant is represented by a chronon. A single chronon may therefore represent multiple instants.