A clock is a system of agreement

An accurate clock is useful only when people understand what its reading means. Two clocks can keep time steadily yet disagree because they were set according to different local conventions. Coordinating a railway, a broadcast, or an international meeting therefore requires more than a reliable mechanism.

[A] Communities must agree on a reference and on how local readings relate to it. [B] Once that convention is established, a difference between clocks can be recognized as an error rather than an alternative definition of the hour. [C] The shared standard allows people who never meet to arrange connected activities. [D] Its value increases as more users rely on the same convention.

Standardization does not remove all practical difficulties. A timetable still needs to make clear which location determines the stated time, and seasonal clock changes can create exceptional cases. Instructions that are obvious to local users may confuse a visitor who follows another convention.

The history of timekeeping is therefore also a history of coordination. Better instruments make measurements more consistent, while social agreements make those measurements usable across a network. The mechanism and the convention solve different problems, and an effective system needs both.