Time
Calculating time intervals
Do not subtract clock times like ordinary numbers. Count on through the next whole hour instead.
Why subtracting goes wrong
10:47 to 11:17 is NOT 11:17 − 10:47 done as a normal subtraction — that gives 70, which is not a time at all. The reason is that time works in 60s, not 10s. Once you pass 59 minutes you go up to the next hour. So use a different method: counting on.
Counting on
Break the journey into easy jumps. From 10:47 to 11:17: 10:47 → 11:00 is 13 minutes 11:00 → 11:17 is 17 minutes 13 + 17 = 30 minutes Jump to the next whole hour first, then on to the finish. It works every time and you never have to borrow.
Longer intervals
For intervals over an hour, count the whole hours as well. From 10:11 to 12:32: 10:11 → 12:11 is 2 hours 12:11 → 12:32 is 21 minutes So the interval is 2 hours 21 minutes. Count whole hours first, then the leftover minutes.
Reading a timetable
A timetable lists the time at each stop, with a column for each service. To find how long a journey takes, find the two times and count on between them. To find how long you must WAIT, count on from your arrival time to the next departure. If you arrive at 09:42 and the train leaves at 10:11, you wait 18 minutes to 10:00 and 11 more — 29 minutes.
Worked examples
Ourtown to Riverton
- The train leaves at 10:11 and arrives at 10:47.
- 10:11 → 10:47 is 36 minutes.
Answer: 36 minutes
Riverton to Hillbury
- Leaves 10:47, arrives 11:17.
- 10:47 → 11:00 is 13 minutes.
- 11:00 → 11:17 is 17 minutes.
Answer: 30 minutes
Ourtown to Newcity
- Leaves 10:11, arrives 12:32.
- 10:11 → 12:11 is 2 hours.
- 12:11 → 12:32 is 21 minutes.
Answer: 2 hours 21 minutes
Waiting for a train
- You arrive at Ourtown at 09:42; the train leaves at 10:11.
- 09:42 → 10:00 is 18 minutes.
- 10:00 → 10:11 is 11 minutes.
Answer: You wait 29 minutes
Why not just subtract?
- 11:17 − 10:47 done as digits gives 70.
- But there are only 60 minutes in an hour.
Answer: Count on instead — the answer is 30 minutes
Where you see this in real life
- Working out how long a train journey takes from a timetable.
- Finding how long until the next bus when you reach the stop.
- Timing how long a film runs from its start and end times.
Key words
- Time interval
- The length of time between two moments.
- Counting on
- Finding a difference by jumping forward in easy steps.
- Timetable
- A table showing when services arrive and depart.
- Duration
- How long something lasts.
- Departure
- The time something leaves.
- Arrival
- The time something gets there.