Time counts in 60s and 24s, not in 10s. Count on in steps to the next hour, read timetables carefully, and add or subtract the time difference when a journey crosses time zones.
To find the time between two times, count on: to the next whole hour, then whole hours, then the minutes left. Never subtract times like ordinary decimals, because an hour has 60 minutes, not 100.
Section 01
12-hour and 24-hour clocks.
Open the lab on 12 and 24 hour clock and drag the time round a full day. Watch how the 24-hour reading keeps counting after midday, while the 12-hour reading starts again and switches from a.m. to p.m.
12-hour clock
24-hour clock
3.15 a.m.
03 15
12.40 a.m. (just after midnight)
00 40
12 noon
12 00
3.15 p.m.
15 15
9.05 p.m.
21 05
For p.m. times after 1 p.m., add 12 to the hours; for 24-hour times after 12 59, subtract 12 and write p.m.
A 24-hour time always has four digits: 7.05 a.m. is 07 05.
Units: 60 s = 1 min, 60 min = 1 h, 24 h = 1 day, 7 days = 1 week, 365 days = 1 year.
Worked example Converting units of time
Write 2.4 days in hours and minutes.
\(2.4 \times 24 = 57.6\) hours.
The decimal part is a fraction of an hour: \(0.6 \times 60 = 36\) minutes.
Answer: 57 hours 36 minutes
Examiner trap
12.40 a.m. is just after MIDNIGHT, so it is 00 40, not 12 40. And 57.6 hours is 57 hours 36 minutes, not 57 hours 60 minutes.
Section 02
Time intervals.
Count on from the start time in friendly steps: first to the next whole hour, then whole hours, then the remaining minutes. In the lab's Time between mode, watch the three jumps on the time line.
Worked example Count on
A delivery van leaves at 08 47 and arrives at 13 22. How long is the journey?
08 47 to 09 00 is 13 minutes.
09 00 to 13 00 is 4 hours.
13 00 to 13 22 is 22 minutes.
Total: 4 hours and \(13 + 22 = 35\) minutes.
Answer: 4 hours 35 minutes
Worked example Across midnight
A night train leaves at 22 48 and arrives at 06 15 the next day. How long is the journey?
22 48 to midnight (24 00) is 1 hour 12 minutes.
Midnight to 06 15 is 6 hours 15 minutes.
Total: \(1\) h \(12\) min \(+ 6\) h \(15\) min.
Answer: 7 hours 27 minutes
Your turn
How many minutes are there from 10 45 to 14 10?
10 45 to 11 00 is 15 min; 11 00 to 14 00 is 3 h; 14 00 to 14 10 is 10 min. Total 3 h 25 min \(= 180 + 25 = 205\) minutes.
Examiner trap
Do not subtract times as decimals: \(13.22 - 8.47 = 4.75\), which is NOT 4 h 75 min or 4.75 hours. The correct interval is 4 h 35 min.
Stage 1 · Learn
Check your understanding
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
Write 4.20 p.m. in the 24-hour clock.
Quick check+10 XP
Write 23 15 in the 12-hour clock.
Quick check+10 XP
How long is it from 09 35 to 11 10?
Quick check+10 XP
How many hours are there in one week?
Section 03
Timetables and time zones.
A timetable lists each journey in a column and each stop in a row. For time zones, find the departure time in the arrival city's time (or the other way round), then add the flight time. The lab's Time zones mode lets you try real routes.
Stop
Bus A
Bus B
Bus C
Station
06 50
07 35
08 20
Market
07 06
07 51
08 36
Hospital
07 23
08 08
08 53
Airport
07 58
08 43
09 28
Worked example Reading a timetable
Using the timetable, Leo arrives at the Market stop at 07 40. He catches the next bus to the Airport. How long does he wait, and when does he reach the Airport?
The next bus from the Market after 07 40 is Bus B at 07 51.
Wait: 07 40 to 07 51 is 11 minutes.
Follow the Bus B column down: Airport 08 43.
Answer: He waits 11 minutes and reaches the Airport at 08 43
Worked example Time zones
A flight leaves London at 21 30 local time and takes 11 hours 50 minutes. Singapore is 8 hours ahead of London. What is the local time in Singapore when it lands?
Arrival in London time: 21 30 + 11 h 50 min = 09 20 the next day.
Singapore is 8 hours ahead: 09 20 + 8 h = 17 20.
Answer: 17 20 (5.20 p.m.) the next day, Singapore time
Examiner trap
'Ahead' means the clock shows a LATER time. When you fly to a place that is ahead, add the time difference to the arrival time in your home time; do not subtract it.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Dubai is 4 hours ahead of London. A flight leaves London at 14 25 local time and takes 6 hours 50 minutes. What is the local time in Dubai when it lands?
Exam style+20 XP
Buses leave the station at 06 50 and then every 45 minutes. The journey to the airport takes 1 hour 8 minutes. Ama arrives at the station at 08 00. When does she reach the airport on the next bus?
Exam style+20 XP
A machine makes one item every 45 seconds. How many items does it make in 6 hours?
Exam style+20 XP
A train leaves at 22 48 and arrives at 06 15 the next morning. How long is the journey?
Skill unlocked
Time, sorted.
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