In North America, the term “military time” usually refers to the use and display of the 24-hour clock. In these countries, the time of day is customarily given almost exclusively using the 12-hour clock notation, which counts the hours of the day as 12, 1, …, 11 with “a.m.” and “p.m.” used.

The 24-hour clock format is commonly used in the United States and Canada only in specific areas such as the military, aviation, navigation, tourism, meteorology, astronomy, computing, logistics, emergency services, and hospitals, where the 12-hour notation can be confusing or even dangerous.

Military usage, as agreed between the United States and NATO military forces, differs in some respects from other 24-hour time systems:

• No hours–minutes separator is used when writing the time, and a letter designating the time zone is appended (for example, 0340Z). See chart.
• Leading zeros are always written and spoken for clarity, so 5:44 a.m. is spoken “zero five forty-four” (casually) or “zero five four four” (on military radio), rather than just “five forty-four”.
• Military time zones use a single-letter suffix after the 24-hour time (for example, 0600R). Each letter corresponds to a specific time zone and is spoken using the NATO phonetic alphabet (R = “Romeo”, Z = “Zulu”, etc.). For example, 6:00 a.m. Eastern Standard Time (UTC−5) is written 0600R and spoken “zero six hundred Romeo”.
• When you want to show local time without specifying an exact time-zone offset, the suffix J (“Juliet”) is used. 1200J (“twelve hundred Juliet”) simply means 12:00 noon local time, wherever “local” happens to be (see chart).
• The suffix Z (“Zulu”) is reserved for Greenwich Mean Time / Coordinated Universal Time (UTC+0). A time such as 1200Z is 12:00 noon at UTC and is spoken “twelve hundred Zulu”. Because Zulu time does not change with daylight saving time, it is often used as a universal reference clock.
• Hours are always spoken in “hundreds”, never “thousands”: 10:00 is “ten hundred”, not “one thousand”; 20:00 is “twenty hundred”.
• 0000 and 2400 are both used for midnight. In many contexts, 0000 refers to midnight at the start of a day and 2400 refers to midnight at the end of a day. Usage depends on service or organizational convention.