Time

When the school bell rings at "1330", do you know if that means lunchtime or home time? Time questions trip students up not because the maths is hard, but because the clock counts in 60s, not 100s — and there is a whole second way of writing the time. Once you see how the 24-hour clock works and how to count a duration, time becomes one of the most reliable marks on the paper.

Parents: ask your child to predict whether a step adds or subtracts hours before they reveal. The "Method tip" boxes name the exact way a PSLE marker rewards a duration calculation.

By the end you'll be able to read the 24-hour clock, find how long something lasts, read a timetable, and solve start-time and end-time problems. Let's begin.

The 24-hour clock

The 24-hour clock writes the time as four digits, like 0830 or 1500. Instead of starting again at 12, it keeps counting: 1 in the afternoon is 1300, 2 is 1400, and so on up to 2359, just before midnight.

For times from 1 pm to 11 pm, add 12 to the hour. So 3 pm becomes 3 + 12 = 15; therefore it is 1500. To go back, subtract 12 from the hour: 20 − 12 = 8; therefore 2000 is 8 pm. Morning times stay almost the same: 7 am is 0700. Noon is 1200 and midnight is 0000, so neither exception needs +12.

12-hour time

6 pm

24-hour time

1800

Reference strip: 0000 midnight · 1200 noon · 6 + 12 = 18

Show the worked solution

The answer is 2100, not "21 pm".

Measuring seconds

A stopwatch often shows minutes:seconds, such as 01:05. The colon separates the minutes from the seconds. One minute is 60 seconds, and one hour is 60 minutes. Read the two fields before finding the elapsed time.

Start display

00:18

minutes : seconds

Finish display

01:05

minutes : seconds

60 s = 1 min · 01:05 = 65 s · 65 s − 18 s = 47 s

Read mm:ss

$$01:05=65\,\text{ s}$$

$$65-18=47\,\text{ s}$$

$$75\,\text{ s}=60\,\text{ s}+15\,\text{ s}=1\,\text{ min }15\,\text{ s}$$

Finding how long something lasts

The time between a start and an end is called the duration. The safe way to find it is to count up in steps: first to the next whole hour, then the full hours, then the leftover minutes.

For example, from 0940 to 1115: count 20 minutes up to 1000, then 1 hour up to 1100, then 15 minutes up to 1115. Total = 1 hour 35 minutes. Counting in friendly steps stops you from trying to subtract 1115 − 0940 like normal numbers, which goes wrong because minutes only reach 60.

Duration = 20 min + 1 h + 15 min = 1 h 35 min

Duration = 20 min + 1 h + 25 min = 1 h 45 min

Reading a timetable

A timetable lines up events with their times. To use one, find the row or column you need, then read across or down to the time. Many questions then ask for the duration between two of those times — so you use the counting-up skill again.

For example, if a bus leaves Town at 0850 and reaches the beach at 0935, the journey takes 45 minutes — counted from 0850 up to 0935. Always read the exact times off the table first, then calculate.

Town to beach timetable

Place	Bus time
Town	08:50
Beach	09:35

08:50 → 09:35 = 45 min

Start-time and end-time problems

Two question types come up again and again. If you know the start time and the duration, add the duration to get the end time. If you know the end time and the duration, subtract the duration to get the start time.

For example, a class starts at 1015 and lasts 1 hour 30 minutes: add 1 hour to reach 1115, then 30 minutes to reach 1145, so it ends at 1145. Working backwards, if a show ends at 2030 and lasted 2 hours, subtract 2 hours to get a start of 1830.

Name the missing time on a timeline

If the finish is 17:10 and the duration is 55 minutes, count backwards in two friendly jumps: 10 minutes to 17:00, then 45 minutes to 16:15. If the start is 09:35 and the duration is 1 h 45 min, count forwards: 1 hour to 10:35, then 45 minutes to 11:20.

Start = 17:10 − 10 min − 45 min = 16:15

Finish = 09:35 + 1 h + 45 min = 11:20

Timeline checks

$$17:10-55\,\text{ min}=16:15$$

$$09:35+1\,\text{ h }45\,\text{ min}=11:20$$

$$13:40\to15:25=1\,\text{ h }45\,\text{ min}$$

Watch out — these are easily mixed up

Quick recap

🎯 Mastery check

Answer all 9 — your progress is saved on this device.

1. What is 9 pm in 24-hour clock time?

   090019002100

2. How long is it from 0815 to 1000?

   1 hour 45 minutes1 hour 15 minutes2 hours 15 minutes

3. A lesson starts at 1130 and lasts 1 hour 20 minutes. What time does it end?

   125012301310

4. A football match ends at 1745 and lasted 90 minutes. What time did it start?

   161516451915

5. A bus leaves at 1255 and arrives at 1340. How long was the journey?

   45 minutes55 minutes1 hour 5 minutes

6. A train timetable shows a train leaving at 0950 and arriving at 1135. A passenger wants the total travel time. What is it?

   Train	Departure	Arrival
   Journey	0950	1135

   1 hour 45 minutes1 hour 85 minutes2 hours 45 minutes

7. A stopwatch changes from 00:18 to 01:05. How many seconds pass?

   47 seconds53 seconds1 minute 23 seconds

8. A show finishes at 17:10 and lasts 55 minutes. What time did it start?

   16:1516:5517:45

9. A lesson starts at 09:35 and lasts 1 h 45 min. What time does it finish?

   10:2011:2011:45

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