0000 military time

In the 24-hour system a day runs from 0000 to 2359, so midnight is written as 0000 — literally "zero hundred hours." It marks the very first minute of a new date, not the last minute of the old one. The US military, ISO 8601, and virtually every computer system treat midnight this way: the clock rolls from 2359 straight to 0000 and the calendar date ticks over.

You will occasionally see 2400 used for midnight, and it is not wrong — but it means something subtly different. 2400 is the end-of-day midnight (the closing bookend of the day that just finished), whereas 0000 is the start-of-day midnight. Because 2400 and the next day's 0000 point to the same instant, most organisations standardise on 0000 to avoid ambiguity. We cover the full distinction on the 0000 vs 2400 page.

Midnight is the single most error-prone moment to record a time, because it is where the calendar date changes. An event logged at 0000 belongs to the new day — a fact that trips up hospital charting, police reports, and shift logs constantly. A medication given "at midnight" is genuinely ambiguous in 12-hour notation (is 12:00 AM the night that just started or the one about to?), which is one reason clinical settings write 0000 and always pair it with the date.

The practical rule: whenever you write 0000, write the date next to it. "0000 on the 5th" is unmistakable; "12 AM" on its own is not.

Computers have used midnight-as-zero since the beginning: Unix timestamps roll a new day at 0000 UTC, and the classic cron schedule 0 0 * * * means "run at 0000 every day." Aviation and NATO operations coordinate in Zulu time, where 0000Z is the reference point the rest of the day is measured from.

Aboard ship, the "mid watch" runs 0000–0400 and is traditionally regarded as the hardest shift to stand — it lands squarely in the body's natural circadian low, when alertness and reaction time bottom out.