The civilian world generally uses a 12 hour clock system, which means the same time notation comes up twice a day, followed by either an AM or PM designation. Asking a friend to meet you at 7:30 for dinner may sound logical, but failure to designate 7:30 PM could create some confusion. The military cannot risk creating confusion in the ranks, so they prefer to use a 24 hour clock to eliminate any ambiguity in time notation.
Military time is divided into hours, beginning with 0000 hours at midnight. From that starting point, the hours increase by 100. One o'clock or 1:00 am in civilian time, for example, would be rendered as 0100 or 0100 hours in military time. This pattern continues with 0200 hours, 0300 hours, 0400 and so on. Minutes after the hour would be rendered in place of the last 00s. 2:45 am would be 0245 hours in military time, and 7:45 am would become 0745 hours.
Noon or 12:00 becomes 1200 hours in military time, and this is where the 24 hour clock notation becomes most noticeable. 1:00 pm become 1300 hours in military time, and begins the pattern of adding to a 1200 hour base. 2:00 pm becomes 1400 hours, followed by 1500 hours (3 pm) and so on until the 2300 hour. The same rules hold true for minutes during the afternoon and nighttime hours. A mission may begin at 1345 hours, for instance. This would be read "thirteen hundred forty-five hours," which would instantly distinguish it from 0145 hours.
The military clock generally follows the local time zone of the installation's location, including daylight savings time adjustments when required. This may require a few calculations if one military base in California is coordinating an operation with a base in Florida. For most events, the given military time refers to the local time zone, so 1345 hours in California may equate to 1645 hours at a base on the east coast.
One exception to this rule involves international operations. Because the world is divided into numerous time zones, the military generally uses Greenwich Mean Time (GMT) when coordinating mission times across several time zones. Because Greenwich, England is in the Z or Zulu time zone, it is not unusual to see the letter Z placed at the end of the 24 hour clock notation. Thus, a mission scheduled to start at 1400Z hours would begin at 2:00 pm local time in Greenwich, England. This "Zulu time" notation is designed to ensure that all military personnel start an operation at precisely the same time.
