Measuring
10 The 24-Hour Clock
Lesson
Learning Outcomes
By the end of this chapter, learners will be able to:
- describe the difference between the ante meridiem (a.m.) and post meridiem (p.m) system of time (12 hour clock) and the 24-hour clock,
- convert time between the 12 and 24-hour clocks, and
- identify the time on an analog clock.
The 24-Hour Clock
In healthcare, the 24-hour clock is often used instead of the Latin system using ante meridiem (a.m.) and post meridiem (p.m.). Using the a.m. and p.m. notation when communicating about time can be confusing and lead to errors in places that function during the night and day. Instead of having time be broken into two periods of 12 hours each, each hour of the day is noted with it’s own number-hence the clock being a 24-hour clock. The 24-hour clock starts counting at 12 a.m., or midnight, at 0000. 1 a.m. is written as 0100, 2 a.m. is written as 0200, 1 p.m. is written as 1300. To determine how to write p.m. times, all you need to do is add 1200 to any notation of p.m. time.
3:00 p.m. = 1500 hours
Eventually, using the 24-hour clock will become habitual, and you will not have to think about how to convert between the two systems at all. You can also refer to the table below as a quick reference to see what each hour of p.m. time is in the 24-hour system. You never have to change how minutes are recorded. Some professions continue to use a colon when writing times using the 24-hour system, but in handwritten healthcare charting the colons are generally left out to keep charting entries clear. Computer systems will often use colons, especially if there are seconds also being recorded.
| Post Meridiem Time | 24-Hour Time |
|---|---|
| 1:00 p.m. | 1300 hours |
| 2:00 p.m. | 1400 hours |
| 3:00 p.m. | 1500 hours |
| 4:00 p.m. | 1600 hours |
| 5:00 p.m. | 1700 hours |
| 6:00 p.m. | 1800 hours |
| 7:00 p.m. | 1900 hours |
| 8:00 p.m. | 2000 hours |
| 9:00 p.m. | 2100 hours |
| 10:00 p.m. | 2200 hours |
| 11:00 p.m. | 2300 hours |
| 12:00 p.m. | 2400 or 0000 hours |
Sample Exercise 10.1
How you write 9:45 p.m. using the 24-hour clock format?
Answer:
945 + 1200 = 2145 hours
You could also use the table and see 9 p.m. = 2100 hours to change the 9 to 21, keeping the minutes the same.
Key Takeaways
- When converting a.m. and p.m. time to the 24-hour system, add 1200 to any p.m. time.
- Notation of minutes remain the same in the 12 and 24-hour systems.
Practice Set 10.1: Converting a.m. and p.m. time to the 24-hour clock
Practice Questions Set 10.1: Converting a.m. and p.m. time to the 24-hour clock
Convert the following times from the a.m. and p.m. system to the 24-hour clock.
- 6:15 a.m.
- 9:30 p.m.
- 4:00 p.m.
- 7:25 p.m.
- 11:15 p.m.
- 2:10 p.m.
- 3:54 a.m.
- 5:05 p.m.
- 1:38 p.m.
- 6:45 p.m.
- 8:45 p.m.
- 9:36 a.m.
- 2:44 p.m.
- 5:12 p.m.
- 10:00 p.m.
- 6:18 p.m.
- 3:48 p.m.
- 11:15 p.m.
- 7:27 p.m.
- 4:20 a.m.
Answers:
- 0615 hours
- 2130 hours
- 1600 hours
- 1925 hours
- 2315 hours
- 1410 hours
- 0354 hours
- 1705 hours
- 1338 hours
- 1845 hours
- 2045 hours
- 0936 hours
- 1444 hours
- 1712 hours
- 2200 hours
- 1818 hours
- 1548 hours
- 2315 hours
- 1927 hours
- 0420 hours
Practice Set 10.2: Converting from the 24-hour system to a.m. and p.m. time
Practice Set 10.2: Converting from the 24-hour system to a.m. and p.m. time
Convert the following times from the 24-hour system to a.m. and p.m. time.
- 1915
- 1435
- 0642
- 1724
- 2317
- 1604
- 1303
- 2159
- 1522
- 2220
Answers:
- 7:15 p.m.
- 2:35 p.m.
- 6:42 a.m.
- 5:24 p.m.
- 11:17 p.m.
- 4:04 p.m.
- 1:03 p.m.
- 9:59 p.m.
- 3:22 p.m.
- 10:20 p.m.
Practice Set 10.3: Converting time on analog clocks to the 24-hour clock
Before noon.
After noon.