24-Hour Clock

24-Hour Clock

24hr clock

When ambiguity in a 12-Hour Clock is considered dangerous, stressful, or uncomfortable and may cause misunderstanding between morning or evening, 24-Hour 
Clock are used in some specialized fields, for example (emergency services, military, aviation, navigation, tourism, meteorology, astronomy, etc.).
In the aviation industry, the 24-Hour Clock is always used in timetables and rosters, and therefore all aviation professionals must be fully conversant with the use of the 24-Hour Clock.

The first use of 24-Hour Clock

In 1956, an American inventor made the first digital clock using 24-hour time. The 24-Hour Clock is always referred to in four (4) digits with no decimal points or mention of a.m. or p.m.
There are 24 hours in a day and the 24-hour clock divides the a.m. from the p.m. at noon and midnight.
Noon is therefore represented by 1200, the 12th hour of the day, and midnight by 2400, the 24th hour of the day.
Minutes are represented by the last two (2) digits. Example: 3.55 p.m. the time on the 24-hour clock would be 1555. When it is the hour after midnight the time will only be in minutes as we have not reached 0100. Therefore, 1230 a.m. would be written as 0030.

12 am in 24 hour clock

The only deviation from this system is at midnight. Midnight is written as 0001 or 2359 so that there is no confusion as to the day of the flight.
Here are a few examples to show you how to convert (change) times from 12- hour clock to 24-hour clock.
24 hour time format

Greenwich Mean Time /Universal Co-ordinated Time

When an aircraft is in flight, it is traveling over various time zones which would create confusion with crew continually altering their watches. The ATC and Flight Scheduling also come upon the problems of local times, so in aviation, we adopt a standard time known as Greenwich Mean Time which is usually written as GMT OR Zulu.
The new international standard time is known as Universal Coordinated Time, abbreviated to UTC , and may be referred to as Zulu time. It is the same time reference as GMT.
GMT is the particular time of the day at the Greenwich Meridian and all the local times are calculated from this. To calculate local times, it depends on how far away from Greenwich you are and whether you are going eastwards or westwards.

24-Hour Clock Examples

Traveling East you have to add on a calculated number of hours to GMT e.g. Tokyo is 9 hours ahead of Greenwich, so if GMT is 1200, local time in Tokyo is 12+9 which makes 2100.
Traveling west from Greenwich you do the opposite thing e.g. Los Angeles is 7 hours behind GMT 1200, local time in Los Angeles is 1200-7 which makes it 0500.


Maged Saeed AL-Hadabi

I’m Instructor / Maged Saeed Al-Hadabi. ​ Air Cargo / IATA Dangerous Goods Regulations / Safety Management System Senior Instructor, Auditor [ Yemen Airways] . Approved IATA DGR/ SMS Instructor by Yemen Civil Aviation Authority. We hope you find Aviation Professional website not only informative, but interesting and helpful as well. Leave your comment , thank you.

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