Difference between time

Question

I have a map of Hours and Minutes that defines the time that something must happen. They are relative to system time (I get the system time using GetLocalTime function). Being m_StartTimes just a simple std::map<int, int>.

I can add several different times like

m_StartTimes[10] = 0; // 10:00 AM
m_StartTimes[15] = 30; // 3:30 PM
m_StartTimes[22] = 0; // 10:00 PM

Then I use this loop to see which event inside of m_Events will be triggered.

for (size_t i = 0; i < m_Events.size(); i++)
{
    for (auto const & c : m_Events[i].m_StartTimes)
    {
        // Time to start the event.
        if (st.wHour == c.first && st.wMinute == c.second)
        {
            // Start the event code.
        }
        else
        {
            // Display time left of nearest time
        }
    }
}

All my trouble is when the time isnt right, it must detect what is the next hour that it will execute. For example: if my system time is 11:00 AM, then it will start next at 3:30PM. If my system time is 8:00 PM, it will start next at 10:00 PM.