Difference between On the clock and On the hour

on the clock—(coll.)

1. said of one’s working time:

• Hey, get to work, you’re still on the clock, stop loafing.

2. on the dial of a taximeter, speedometer, etc.:

• Neither vehicle had much mileage on the clock.

on the hour—at exactly one o’clock, two o’clock, etc.:

• Buses for London leave every hour on the hour.

See also: in time / on time.