struct ClassTime
{
    void operator++();
    void operator--();

    unsigned hour;
    bool     half;
};

size_t operator-(const ClassTime& a, const ClassTime& b);

This structure represents a time of day when a class can begin or end. The resolution of this structure is currently 30 minutes.

unsigned hour;

The hour of the day. Can range from 9 to 21.

bool half;

False if it's on the hour, true if it's 30 minutes after the hour.

void operator++();

Increments this object by 30 minutes.

void operator--();

Decrements this object by 30 minutes.

size_t operator-(const ClassTime& a, const ClassTime& b);

The number of time units from b to a. Precondition: a is not earlier than b.