The key distinction is functional: time-of-day recording apparatus is designed to capture, print, or log timestamps and time intervals — such as time clocks used for employee attendance or process timing — rather than simply display the current time. The presence of a clock or watch movement or synchronous motor is a required characteristic, but the recording or measuring output is what separates these devices from decorative or display clocks.

Combination devices can be difficult to classify because they share features with pure clocks, time switches, and recording instruments. Customs authorities generally classify by the device's principal function; if the recording or interval-measuring function is primary, it falls here rather than under clock or time-switch headings. Consulting a licensed customs broker before entry is advisable for multi-function units.