The 24-hour clock is a convention of time keeping in which the ay runs from midnight to midnight and is divided into 24 hours, numbered from 0 to 23. This system is the most commonly used time notation in the world today. The 12-hour clock is dominant instead in a handful of countries, particularly the United States and Canada (except Quebec). 24-hour notation is also popularly referred to as military time or astronomical time in the US and Canada, and in Australia (though rarely) as army time. In some parts of the world, it is called railway time. It is also the international standard notation of time (ISO 8601).