The 24-hour clock is a way of telling the time in which the day runs from midnight to midnight and is divided into 24 hours, numbered from 0 to 24. It does not use a.m. or p.m. This system is also referred to (only in the US and the English speaking parts of Canada) as military time or (only in the United Kingdom and now very rarely) as continental time. In some parts of the world, it is called railway time. Also, the international standard notation of time (ISO 8601) is based on this format.