24H

24h, shorthand for twenty-four hours, denotes a period of one day. In civil timekeeping, the day is commonly divided into 24 hours, and the hour is the base unit of time in everyday use. An hour equals 3,600 seconds, so 24 hours equal 86,400 seconds. The length of a calendar day is nominally a mean solar day, but real days vary due to Earth's rotation, so adjustments such as leap seconds are occasionally added to maintain alignment with atomic time (UTC). The unit hour is defined in the International System of Units as a derived unit, with the second as the base unit.

Time notation: The 24-hour clock expresses the day from 00:00 to 23:59, avoiding AM/PM. It is common in many regions for schedules, transport timetables, computing, and military usage; in everyday language, "h" is used as an abbreviation for hour, as in "9 h" or "24 h".

Other uses: In certain contexts, 24h denotes a duration—e.g., a process lasting 24 hours. The concept also appears in cultural and engineering contexts, such as "24h Le Mans" endurance racing or service availability labels like "24h support".