timekeeper

noun

1. A device that shows the time; a timepiece.

   “The sailors sail by chronometers that do not lose two or three seconds in a year, ever since [Isaac] Newton explained to Parliament that the way to improve navigation was to get good watches, and to offer public premiums for a better time-keeper than any then in use.”

2. A person who keeps records of the hours of attendance of employees.
3. A person who records the time elapsed in a sporting event.
4. The group member who controls the rhythm of the music when a group of musicians play together.

   “There was no consensus among jazz musicians about who was the primary timekeeper in a jazz band; some said the drummers and others said the bassist.”

5. (usually)A person (or something controlled by a person) that is punctual.

   “As it happens, the Leicester–Beeston train is quite a good timekeeper—it sometimes runs a few minutes early— […]”