Time Intervals
Measure · Year 3–Year 6
How long from one time to another, one jump at a time.
About the time intervals
Set a start time and an end time on the board, and the jumps between them appear one press at a time: up to the next whole hour, then the whole hours, then the minutes left over. Each jump gets an arc, a length and a sentence, and the total arrives with the last one.
11:20 take away 9:45 is not a column subtraction, and that is the whole reason this is hard. There are sixty minutes in an hour rather than a hundred, so a class who line the digits up and subtract get 1.75 of something. Bridging through the hour is the method that works, and it is what the board draws.
It opens on the question rather than the answer. Nothing is on the line until somebody presses Next, so the class can be asked how many minutes it is to the next hour before the board says. The last press shows the last jump AND the total, because the last jump is the one a class is still being asked about.
Changing a time throws the working away, because the jumps are worked out from the two times rather than stored. A board where the arcs and the times disagree is not reachable.
The whole answer is written in words: 1 hour and 35 minutes, never 1:35. A duration written like a time is the mistake waiting on the other side of this, and a class who reads their answer off a clock face has learned the wrong thing.
How to use it on your whiteboard
- 1Set the two times with the chevrons above and below each part.
- 2Ask the class how long it is to the next whole hour.
- 3Press Next. The first jump appears with its length and a sentence.
- 4Keep going: whole hours, then the minutes left over. The total arrives with the last jump.
- 5Tap 12 or 24 to switch the clock, which changes nothing about the arithmetic.
Questions
- Why not just subtract the two times?
- Because time is not base ten. 11:20 minus 9:45 done in columns gives 1.75, which is not an hour and 35 minutes. Bridging through the hour keeps every step in minutes or in whole hours, which are the two things a class can count.
- Why does the answer say 1 hour and 35 minutes rather than 1:35?
- Because 1:35 is a time and this is a length of time. Writing a duration as a clock time is the next mistake along, and it is worth being strict about it on the board.
- How small a step can the times take?
- Five minutes, which is every mark on a clock face. The hour chevron moves a whole hour, and both of them add time: five minutes on from 9:55 is 10:00.
- Does it work with a 24 hour clock?
- Yes. The 12 and 24 buttons change how the times are written and nothing else, so the same interval can be read both ways, which is the Year 4 objective about converting between them.