This is a brief explanation of the “24 Hour Clock” also known as “Military Time”. It is the most widely used format of time keeping in the world today, and I would highly advise anyone who doesn’t already know how to use it, to learn it, use it and make it a part of your life. Set your vehicle clock, alarm clocks, cell phone clock, computer clock or any other digital device you have to the 24 hour time setting to become familiar with it. If you are into astronomy and follow current events in astronomy you will time and time again find them in the user friendly 24 Hour/Military Time format. It truly is a simple concept and is not hard to learn at all as it’s simply 24 hours in a day and the hours run 00:00 through 23:59:59.
All of the posts I update the BLOG with are and will be in military time. It’s a simple way of detailing time and leaves no mistake as to whether I mean AM or PM in a given circumstance. Keep in mind this is just the time keeping format, I use Military time coupled with Coordinated Universal Time (UTC) to deliver standard baseline times that anyone around the world can easily understand (I will explain UTC in a separate page). Below I show the 24 Hour/Military time scale with a couple of examples as to how the times are spoken as well as a short point to be made on the 0000 vs. 2400 time designation for midnight.
24 hr. = 12 hr. (Note that the change doesn’t significantly change until 1pm)
01:00 = 1 am
02:00 = 2 am
03:00 = 3 am
04:00 = 4 am
05:00 = 5 am
06:00 = 6 am and is spoken as: “Zero Six Hundred Hours” not “OH Six Hundred Hours”.
07:00 = 7 am
08:00 = 8 am
09:00 = 9 am
10:00 = 10am
11:00 = 11am
12:00 = 12 NOON
13:00 = 1 pm
14:00 = 2 pm
15:00 = 3 pm
16:00 = 4 pm
17:00 = 5 pm
18:00 = 6 pm and is spoken as: “Eighteen Hundred Hours”
19:00 = 7 pm
20:00 = 8 pm
21:00 = 9 pm
22:00 = 10pm
23:00 = 11pm
00:00 or 2400 = 12 MIDNIGHT (explanation below)
It is acceptable to use 24:00 vs. 00:00 as midnight when it is used to document or record an end of the day, though the standard is always 00:00.
KEEP IN MIND….
The rest of the midnight hour is expressed in a “00” designation not a “24” designation:
12:15 am = 00:15 not “24:15”
12:45 am = 00:45 not “24:45”
Some documents and or procedures use 00:00 as midnight when starting a day and 24:00 as midnight when ending a day, no idea why that would be necessary as 00:00 and 24:00 both clearly mean midnight and as the rest of the hour is in “00” format anyway. Personally I never use 24:00 as it is unnecessary and influences one to (incorrectly) use the “24” designation for the rest of that hour.
Another note of interest: The 24hr clock is always in the HH:MM:SS:ss (sub-seconds) format and you do not have to use the “colons” between the hours and minutes unless you are also counting seconds and sub-seconds then it is helpful to place colons between designations.
10:30pm can be designated as 2230 unless you are counting seconds then you would designate it 22:30:15 (or whatever the seconds happen to be that you are counting).
Some organizations never use colons and that’s acceptable because the first two digits are always hour, the second two are always minute and the third set is always seconds ex: 223015.
You may alternatively choose to always keep the colon between designations even when only annotating hours and minutes; that is fine as well ex: 22:30. Many find this method best because it always keeps things uniform and in a familiar pattern that’s easily discernible.
I hope you understand and find this page useful and as always please ask if you ever come across a question you need to have clarified.
US NAVY Astronomical Info Center: http://aa.usno.navy.mil/faq/