Time-of-flight is a period of time between a gun's discharge and the impact of the projectile against land, sea, or other object.
Generally, time-of-flight is considered as that when a shell would strike the water, as in all but the shortest range firings the abbreviation of a projectile's flight by hitting a vessel before it reaches the water is meager indeed, and in all cases the distinction is largely mooted by the fact that you hit the target and time would be better spent sipping port rather than discussing technicalities of fire control.
Due to aerodynamic drag, time-of-flight generally increases more and more quickly as range increases.
By 1908, the Royal Navy was trialling time-of-flight watches to assist spotters in identifying their own shell fall. The watch would be set to the given gun range, started when the guns fired, and would ring a bell at the moment the shells should fall.
- Fire Control, 1908, p. 5.