Modular arithmetic might be a new concept, so I'll outline it very briefly.
In modular arithmetic, all numbers are reduced to a smaller cyclic subset of integers by the process of dividing by the modulus and taking the remainder. So if you count modulo four starting at zero, you begin normally:
0, 1, 2, 3
When you reach 4, the modulus, you go back to zero, because 4 divided by 4 is 1, with remainder 0. So this restarts the cycle.
For your problem, the hours of the day are modulo 24 - and you can probably see how this applies from there.