For example, when computing the time of day using a 12-hour clock obtained by adding four hours to 9:00, the answer, 1:00, is obtained by taking 9+4=1 (mod 12) (i.e., adding the hours with modulus 12).

>>519504

modular arithmetic, also called clock arithmetic.

>>519553

www.khanacademy.org/computing/computer-science/cryptography/modarithmetic/a/what-is-modular-arithmetic