You probably do this all the time with time without realising it.

If it’s 11 o’clock and we agree to meet in two hours, you instinctively know I’m talking about 1 o’clock. But what you’ve just done is add two numbers mod(ulus) 12.

In effect your brain added 11+2 to get 13, then took away 12 to make it fit, and came up with the modulus (or remainder) of 1.

Modulus arithmetic is therefore sometimes known as clock arithmetic.