The 12-hour and 24-hour Clock

15 min

Let's begin

Umutoni is travelling to Rubavu to visit her grandmother. Her bus ticket says Departure 14:30. Her little brother reads it and asks, "Does the bus leave at half past two in the middle of the night?" Umutoni laughs. She knows how to read the 24-hour clock, and after this lesson you will too.

Two ways to write the time

A day has 24 hours. The 12-hour clock counts from 1 to 12 twice and adds a.m. (from midnight to noon) or p.m. (from noon to midnight). The 24-hour clock counts the hours straight from 00 to 23 and always uses four digits, like 07:15 or 19:40. It needs no a.m. or p.m., which is why buses, airports and phones use it.

From the 12-hour clock to the 24-hour clock

a.m. times keep their hours; just write four digits. 7:15 a.m. = 07:15.
p.m. times: add 12 to the hours. 2:30 p.m. = 2 + 12 = 14:30. 9:45 p.m. = 21:45.
Take care with 12 o'clock. 12:20 p.m. (just after noon) = 12:20. 12:20 a.m. (just after midnight) = 00:20.

From the 24-hour clock to the 12-hour clock

If the hours are 13 or more, subtract 12 and write p.m. 18:05 = 18 − 12 = 6:05 p.m.
If the hours are from 01 to 11, write a.m. 07:05 = 7:05 a.m.
00:40 = 12:40 a.m., and 12:00 = 12 noon.

So Umutoni's bus leaves at 14:30 = 2:30 p.m., in the afternoon.

How long does a journey take?

Worked example

A bus leaves Kigali at 10:40 and reaches Huye at 13:25. How long is the journey?

Count on in easy jumps:
10:40 → 11:00 is 20 minutes.
11:00 → 13:00 is 2 hours.
13:00 → 13:25 is 25 minutes.
Total: 2 hours + 20 minutes + 25 minutes = 2 h 45 min.

Worked example

(through midnight). A night bus leaves at 22:30 and arrives at 05:15 the next morning.
22:30 → 24:00 (midnight) is 1 h 30 min.
00:00 → 05:15 is 5 h 15 min.
Total: 6 h 45 min.

Worked example

(adding time). A journey of 3 h 50 min starts at 09:25. When does it end?
09:25 + 3 h = 12:25. Then 12:25 + 50 min = 13:15.

Tip

After noon, add 12. Before noon, keep the hours. And remember that midnight starts the day at 00:00.

✦

Try it yourself

Write 4:50 p.m. in the 24-hour clock.
Write 21:15 in the 12-hour clock.
Write 07:05 in the 12-hour clock.
How long is it from 08:45 to 11:20?
How long is it from 23:10 to 01:40 the next day?

Answers

16:50.
9:15 p.m.
7:05 a.m.
08:45 → 09:00 is 15 min, 09:00 → 11:00 is 2 h, 11:00 → 11:20 is 20 min. Total 2 h 35 min.
23:10 → 00:00 is 50 min, and 00:00 → 01:40 is 1 h 40 min. Total 2 h 30 min.

Quick check

What you learned today

The 24-hour clock runs from 00:00 to 23:59 and needs no a.m. or p.m.
For p.m. times, add 12 to the hours; for times after 12:59, subtract 12 and write p.m.
Find how long something takes by counting on in easy jumps.

Sources and credits

Written by Ganzaa.

1Two ways to write the time

2From the 12-hour clock to the 24-hour clock

3From the 24-hour clock to the 12-hour clock

4How long does a journey take?

Try it yourself

Two ways to write the time

From the 12-hour clock to the 24-hour clock

From the 24-hour clock to the 12-hour clock

How long does a journey take?