When I look at the task, it looks more like C than C ++.
As Rick York already wrote, one would actually solve that with a class.
However, the given framework does not fit.
So a structure like this is needed to define time:
typedef struct {
int h, m, s;
}Time;
Since the prototypes for the functions are already available, it shouldn't be a problem to calculate the difference between two times.
Realizing the difference calculation with seconds as suggested makes sense, since the result should also be time according to the task. Here 4Byte should be able to calculate if max. 24h:59min:59sec can come out.