There are two main methods of displaying the time. The first one is the 12-hour clock which uses AM and PM, and the other is the 24-hour clock.
In 12-hour clock or 12-hour time notation the day is split into two 12-hour periods running from midnight (12:00 AM) to noon (12:00 PM), and noon to midnight.
In military time the day runs from midnight to midnight and is divided into 24 hours from 00:00 (midnight) to 23:59.
Military time or 12-hour military notation consists always of four digits. It starts at 0000, which corresponds to 12:00 AM. Thus, 12:00 AM = 0000 (subtract 12-hours). Thus, 12:15 AM = 0015, 12:30 AM = 0030 and so on. After 12:59 AM the 12-hour clock jumps to 1:00 AM = 0100 in military time. So, 1:30 AM = 0130, 9:30 AM = 0930, 11:20 AM = 1120, 12:59 PM = 1259 in military time. Within these intervals above the hours and minutes remain the same.
After and including 1:00 PM until 11:59 PM, you should add 12 to the hours to make the corresponding military time. For example: 2:00 PM = (2 + 12):00 = 1400, 5:30 PM = (5 + 12):30 = 1730, 11:59 PM = (11 + 12) = 2359 in military time.
After 23:59 PM the 12-hour clock turns to 12:00 AM which corresponds to midnight, at which point military time is reset to 0000 (the beginning of the day) and the process continues.
The math to convert from AM/PM (12-hour clock) to 24-hour clock is the same explained above for military time with small diferences in the way we write or spell the time. For example: 1:45 AM = 1:45, 10:35 AM = 10:35, 12:35 AM = 0:35, 3:33 PM = 15:33 in 24-hour time, and so on. Keep in mind that the 12-hour format is the one that appears on a actual watch that you wear on your wrist. So, if the hour value has only 1 digit, the clock shows only 1 digit.
When converting from AM/PM to military time or to 24-hour time, you just need to change the hour values while minute values remain the same.
From 12:00 AM to 12:59 AM, subtract 12 hours:
From 1:00 AM to 12:59 PM, the hours and minutes remain the same:
From 1:00 PM to 11:59 PM, add 12 hours:
The abbreviations AM and PM are from Latin:
In military time hours are always "hundred", never "thousand", so
12:00 AM = 00:00 = midnight; 12:00 PM = 12:00 = noon.
In the 24-hour time notation, the day begins at midnight, 00:00. The notation 24:00 serves to refer to the end of a day. Physically, 24:00 of one day is the same time as 00:00 of the following day.
|12:00 AM||0000||zero hundred hours|
|00:01 AM||0001||zero zero zero one hours|
|12:30 AM||0030||zero thirty hours|
|1:00 AM||0100||zero one hundred hours|
|1:30 AM||0130||zero one thirty hours|
|2:00 AM||0200||zero two hundred hours|
|2:15 AM||0215||zero two fifteen hours|
|3:00 AM||0300||zero three hundred hours|
|3:20 AM||0320||zero three twenty hours|
|3:45 AM||0345||zero three forty-five hours|
|4:00 AM||0400||zero four hundred hours|
|4:35 AM||0435||zero four thirty-five hours|
|5:00 AM||0500||zero five hundred hours|
|6:00 AM||0600||zero six hundred hours|
|7:00 AM||0700||zero seven hundred hours|
|8:00 AM||0800||zero eight hundred hours|
|9:00 AM||0900||zero nine hundred hours|
|10:00 AM||1000||ten hundred hours|
|11:00 AM||1100||eleven hundred hours|
|12:00 PM||1200||twelve hundred hours|
|1:00 PM||1300||thirteen hundred hours|
|2:00 PM||1400||fourteen hundred hours|
|3:00 PM||1500||fifteen hundred hours|
|4:00 PM||1600||sixteen hundred hours|
|5:00 PM||1700||seventeen hundred hours|
|6:00 PM||1800||eighteen hundred hours|
|7:00 PM||1900||nineteen hundred hours|
|8:00 PM||2000||twenty hundred hours|
|9:00 PM||2100||twenty-one hundred hours|
|10:00 PM||2200||twenty-two hundred hours|
|11:00 PM||2300||twenty-three hundred hours|
Whole hours are read "hundred hours". For example: 0200 is spelled "zero two hundred hours" or "oh two hundred hours", 0800 is spelled "oh eight hundred hours", 0905 = "oh nine oh five hours".
For times other than whole hours, the word hundred is omitted and oh or zero is used instead.
In the military time, the word "hours" is used after the numerals for time.