2400 Military Time

2400 is midnight, the moment the date changes. The 12-hour clock calls it 12:00 AM. In 24-hour notation it is 24:00, and the reason it exists at all is that a day that runs from 0000 has to end somewhere: it ends at 2400, one minute after 2359. The next minute is 0001 of the following day.

So 2400 and 0000 are not two different times. They are the same instant written from two sides. A shift that “ends at 2400 Monday” and one that “ends at 0000 Tuesday” end at the same moment; the first form keeps the shift inside Monday, which is usually what the roster wants.

It is spoken “twenty-four hundred”. Nobody says “twenty-four hundred thirty”, because 2430 does not exist: anything after 2400 belongs to the new day and starts over at 0001.

The US services disagree, which is why the question keeps coming up. Army Regulation 25-50, the Army’s correspondence manual, says military time is expressed in four digits from 0001 to 2400. Under that rule the day ends at 2400 and 0000 is never written; the first minute of a day is 0001. The Navy’s correspondence manual (SECNAV M-5216.5) runs the day from 0000 to 2359, so a Navy document never writes 2400 at all.

Outside the military the same split exists. Railway and airline timetables write 2400 for an arrival at the end of the day and 0000 for a departure at the start of it. The older editions of ISO 8601 allowed 24:00 as the end of a day; the current edition drops it for data interchange, and most software treats 24:00 as 00:00 of the next day or rejects it. In a spreadsheet, enter midnight as 0:00 of the next date rather than 24:00.

A practical rule that satisfies everyone: use 0000 for anything that begins at midnight, 2400 for anything that ends at midnight, and 2359 or 0001 when a document must avoid the ambiguity altogether. Nobody will misread any of the three.

A night shift from 1600 to 2400 is eight hours, and the arithmetic is simple because 24 − 16 = 8. Write the same shift as 1600 to 0000 and a naive subtraction gives −16; you have to add 24 to the end time first. That is the whole reason 2400 survives on rosters.

A shift that crosses midnight, say 2200 to 0600, cannot use 2400 at all. The end time is on the next day, and the duration is (0600 + 2400) − 2200 = 8 hours. The work hours calculator applies the midnight rule automatically whenever the end time is earlier than the start time, and the time duration calculator accepts dates on both ends if the shift is longer than a day.

On a time card, the safest entry for a clock-out at midnight is 12:00 AM on the following date, or 24:00 on the day the shift started if the system accepts it. Payroll software that rejects 24:00 will usually accept 23:59, which costs one minute and avoids an argument.

“Twenty-four hundred”, with or without “hours” after it. 0000 is “zero hundred”, and plain “midnight” is understood everywhere. Over the radio, where the day boundary matters, people often avoid both and say “twenty-three fifty-nine” or “zero zero zero one” to make the date unambiguous.

Midnight has a second spelling, 2400, used when it marks the end of a day rather than the start. The full rules, with a converter, are on the military time chart page.