The 24-hour clock was a method of measuring time based on the 24 hours of the Earth day. It ran from midnight to midnight, starting at 0 hundred and ending at 24 hundred. A hundred was another word for hour. This was the standard time notation aboard Federation starships of the 23rd and 24th centuries. (TOS: "The Cage")
In 2254, on board the USS Enterprise, Captain Christopher Pike wanted the reports at oh five hundred (05:00). (TOS: "The Cage")