Chronometer

From Memory Alpha, the free Star Trek reference.

A chronometer is a device used to keep track of time. Starships measure the passage of time when they traverse the space-time continuum other than Albert Einstein's proper time by use of chronometers. (TOS: "The Naked Time", "Tomorrow is Yesterday")

On occasion, starships may need to be reset the on-board chronometer, at which time a Federation time beacon may be consulted. (TNG: "Cause and Effect")

In "The Naked Time", the original version featured a mechanical chronometer. In the remastered version, it was changed to a digital display.