The 24-hour clock is the convention of time keeping in which the hours passed since midnight run from 0 to 23. It is popularly referred to as military time in the United States, Canada, and a handful of other countries where the 12-hour clock is still dominant.
I found this example of a railway timetable showing both 00:00 and 24:00 on Wikipedia. This timetable has a pleasant retro style and the curious concept of what you might call a “25-hour clock”. That is, the schedule uses the hours 1 through 24, plus zero.