Solving Problems with 12- and 24-Hour Time
Year 5 · Maths · Free — no account needed
Solving Problems with 12- and 24-Hour Time
Two ways to write time
12-hour time uses the numbers 1 to 12, with "am" (before midday) or "pm" (after midday) to show which half of the day it is. 24-hour time uses four digits from 0000 to 2359, with no am or pm needed, because the hours just keep counting up past 12.
Converting 12-hour to 24-hour
For am times from 1 am to 11 am, drop the "am" and write the hour with a leading zero if needed: 7:15 am becomes 07:15. The 12 am hour is the exception: it becomes 00, so 12:15 am becomes 00:15. For pm times (except 12 pm), add 12 to the hour: 3:40 pm becomes 15:40, because 3 + 12 = 15.
Converting 24-hour to 12-hour
From 0000 to 1159, it is morning: add "am" (0900 becomes 9:00 am; 0000 to 0059 becomes 12:00 am to 12:59 am). From 1200 to 1259, it is just after midday: keep the hour and add "pm" (1230 becomes 12:30 pm). From 1300 to 2359, subtract 12 from the hour and add "pm": 1745 becomes 5:45 pm, because 17 − 12 = 5.
Midday and midnight
Midday (12 noon) is written as 12:00 pm in 12-hour time, and as 12:00 in 24-hour time. Midnight is written as 12:00 am in 12-hour time, and as 00:00 in 24-hour time — never 24:00.
Working out elapsed time
To find how much time has passed, it helps to break the journey into parts either side of midday or midnight, rather than subtracting straight across. Count the minutes to the next whole hour or to midday/midnight, then add the whole hours, then add any leftover minutes.
Reading timetables
Timetables usually use 24-hour time to avoid mixing up am and pm. Read the departure and arrival times carefully, and check whether a trip crosses midday or midnight before working out how long it takes.
Worked Example
A ferry departs at 10:40 pm and arrives at 1:15 am the next day. How long is the trip? Step 1: Convert both times to 24-hour time so there is no am/pm confusion: 10:40 pm = 22:40, and 1:15 am = 01:15. Step 2: Work out the time from 22:40 to midnight (00:00). From 22:40 to 23:00 is 20 minutes, then 23:00 to 00:00 is 1 more hour — that's 1 hour 20 minutes. Step 3: Work out the time from midnight to 01:15 — that's 1 hour 15 minutes. Step 4: Add the two parts together: 1 hour 20 minutes + 1 hour 15 minutes = 2 hours 35 minutes.
Practice
Convert 4:35 pm to 24-hour time.
Flashcards
What does "am" mean?
Any time from midnight up to (but not including) midday.
What does "pm" mean?
Any time from midday up to (but not including) midnight.
How do you convert an am 12-hour time to 24-hour time?
Drop the "am" and write the hour with a leading zero if needed, e.g. 7:15 am = 07:15. The 12 am hour becomes 00: 12:15 am = 00:15.
How do you convert a pm time (except 12 pm) to 24-hour time?
Add 12 to the hour, e.g. 3:40 pm = 15:40 because 3 + 12 = 15.
How do you convert a 24-hour time of 1300 or more back to 12-hour time?
Subtract 12 from the hour and add "pm", e.g. 1745 = 5:45 pm because 17 − 12 = 5.
How is midday written in 24-hour time, and how is midnight written?
Midday is 12:00; midnight is 00:00 (never 24:00).
What is a good strategy for elapsed time that crosses midday or midnight?
Break the time into parts either side of midday/midnight, then add the parts together.
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