Clock arithmetic, also known as modular arithmetic, is a system of arithmetic for integers that wraps around upon reaching a certain value, called the modulus. For example, in a 12-hour clock, after reaching 12, the count starts again at 1. This means that 13 o'clock is equivalent to 1 o'clock, and 14 o'clock is equivalent to 2 o'clock. In clock arithmetic, addition and subtraction are performed similarly to regular arithmetic, but the results are adjusted to fit within the range defined by the modulus. For instance, adding 9 and 5 in mod 12 results in 2, since 14 exceeds 12 and wraps around. This concept is useful in various fields, including computer science and cryptography.