Clock arithmetic is a familiar term for modular arithmetic. It refers to the behaviour of addition of hours on a clock face: for example, if we begin at 7 o'clock and add 8 hours, then rather than ending at 15 o'clock (as in usual addition), we are at 3 o'clock.
Likewise, if we start at noon and count off 7 hours three times (), we end up at 9 o'clock (rather than 21).
Essentially, when we reach 12, we start over; 12 is called the modulus, hence the name "modular arithmetic."
We can easily pretend that the clock face contains any number of hours, and calculate according to the new modulus.
