On the clock is an idiom that has been in use for about 100 years. We will examine the meaning of the idiom on the clock, where it came from, and some examples of its idiomatic usage in sentences.
On the clock is an idiom that is used to mean that someone is working, that someone is currently performing duties for an employer. The expression on the clock is a reference to punching a time clock, which is an apparatus that keeps track of how long one is at work. The idiom on the clock may apply to working at a job that requires using a time clock or it may be applied to a job that does not require using a time clock. The phrase on the clock came into use in the early 1900s, when time clocks were installed in factories and other places of work. On the clock is sometimes used to mean that someone is running out of time.
Investigators dropped him as a suspect because of his alibi that he was on the clock at a Brunswick Winn-Dixie. (The Atlanta Journal Constitution)
They found him falsifying time cards, claiming he was on the clock while dining, commuting, and “meeting with women.” (New York Magazine)
A Circus Circus maintenance worker was arrested in July after a hotel guest accused him of raping her while he was on the clock, the Las Vegas Review-Journal has learned. (The Las Vegas Journal-Review)