A time clock is a mechanical device that was used to assist in tracking the hours an employee of a company worked. This was accomplished by inserting a heavy paper card, called a time card , into a slot on the time clock. When the time card hit a contact at the rear of the slot, the machine would print day and time information on the card. This allowed a time keeper to have an official record of the hours an employee worked to calculate and pay an employee.

The time cards usually had the workdays and time in and time out areas marked on them so that employees could "punch in" or "punch out' in the correct place. The employee was responsible to line up the correct area of the card to be punched with an indicator on the time card.

The Poughkeepsie National Guard Armory had on display one of the oldest operating time clocks, as of 1980.

In the 1990's, time clocks started to move away from the mechanical version to computer based time tracking systems. On these systems, the employee either enters an employee number, swipes a magnetic striped card or uses some other device to identify the employee to the system. Based on the system being used, the employee then enters what should be recorded. This could be "punching in", "punching out", lunch breaks, reason for leaving early, or any other type information the employer requires.