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Year 4 Maths Time

Duration & elapsed time

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Theory

Duration and elapsed time is a Year 4 time skill: finding how much time passes between a start time and a finish time. This page shows how to count on along a timeline, bridge the hour, and count whole hours and then minutes, using the fact that 60 minutes make 1 hour. Describing how long events take starts in Year 3 (VC2M3M03 / AC9M3M03); finding durations is Year 4. Curriculum: VC2M4M03 / AC9M4M03.

Duration means how long something lasts. Elapsed time is the amount of time that passes between a start time and a finish time. For example, if a lesson starts at \(9\!:\!10\) and finishes at \(9\!:\!45\), the elapsed time is 35 minutes.

To find elapsed time we count on from the start to the finish along a timeline. We jump in easy steps: first the minutes up to the next o'clock, then the whole hours, then the last minutes. This is called bridging the hour.

Time is measured in hours and minutes. There are 60 minutes in one hour, so time does not work like ordinary counting to 100. That is why we count on instead of subtracting the digits.

Count-on number line from 3:45 to 4:20 A timeline showing the times 3:45, 4:00 and 4:20 with two labelled jumps, plus 15 minutes then plus 20 minutes, counting on to make 35 minutes. 3:45 4:00 4:20 +15 min +20 min
Bridge the hour: 15 min to 4:00, then 20 min more makes 35 minutes.
Count-on number line from 9:40 to 11:10 A timeline showing the times 9:40, 10:00, 11:00 and 11:10 with three labelled jumps, plus 20 minutes, plus 1 hour and plus 10 minutes, counting on to make 1 hour 30 minutes. 9:40 10:00 11:00 11:10 +20 min +1 hour +10 min
Hours and minutes: 20 min, then 1 hour, then 10 min makes 1 hour 30 minutes.

Elapsed time does not need a formula, but these time facts help you count on quickly.

\[ 60 \text{ minutes} = 1 \text{ hour} \]
60 minutes =1 hour
\[ 30 \text{ minutes} = \text{half an hour} \]
30 minutes = half an hour
\[ 15 \text{ minutes} = \text{quarter of an hour} \]
15 minutes = quarter of an hour
Count on, do not subtract. Because 1 hour is 60 minutes, you cannot subtract the digits. Count on from the start time in jumps: minutes to the next o'clock, then hours, then the last minutes.
\[ 1 \text{ hour } 15 \text{ minutes} = 60 + 15 = 75 \text{ minutes} \]
1 hour 15 minutes = 75 minutes
Start or finish time? To find a finish time, count on from the start. To find a start time, count back from the finish, bridging the o'clock on the way.

How to find elapsed time by counting on

  1. Start at the start time. Put the start time on the left of a timeline and the finish time on the right.
  2. Bridge the hour. Count the minutes from the start up to the next o'clock. That is your first jump.
  3. Count the hours. Jump on one hour at a time until you reach the o'clock just before the finish time.
  4. Count the last minutes. Jump the last few minutes to land on the finish time.
  5. Add the jumps. Add all the jumps together to get the total elapsed time.

How to find a finish time or a start time

  1. Finish time: start at the start time and count on the duration, first up to the next o'clock, then the rest.
  2. Start time: start at the finish time and count back the duration, first back to the o'clock, then the rest.
  3. Hours and minutes: 60 minutes make 1 hour, so 70 minutes is 1 hour with 10 minutes left over.
Example 1 — Minutes in one hour
Silent reading runs from \(2\!:\!20\) to \(2\!:\!50\). How long is it?
Solution
\(\text{same hour}\)\(\to\)\(\text{count the minutes}\)
\(50 - 20\)\(=\)\(30\)
50-20=30

The reading lasts \(30\) minutes.

Example 2 — Bridge the hour
A bike ride starts at \(3\!:\!45\) and ends at \(4\!:\!20\). How long is it?
Solution
\(3\!:\!45 \to 4\!:\!00\)\(=\)\(15\text{ min}\)
\(4\!:\!00 \to 4\!:\!20\)\(=\)\(20\text{ min}\)
\(15 + 20\)\(=\)\(35\text{ min}\)
15+20=35

The ride lasts \(35\) minutes.

Example 3 — Read a timeline
A ferry leaves at \(11\!:\!40\) and arrives at \(12\!:\!15\). Count on to find how long the trip takes.
Count-on number line from 11:40 to 12:15 A timeline showing the times 11:40, 12:00 and 12:15 with two labelled jumps, plus 20 minutes then plus 15 minutes, counting on to make 35 minutes. 11:40 12:00 12:15 +20 min +15 min
Solution
\(11\!:\!40 \to 12\!:\!00\)\(=\)\(20\text{ min}\)
\(12\!:\!00 \to 12\!:\!15\)\(=\)\(15\text{ min}\)
\(20 + 15\)\(=\)\(35\text{ min}\)
20 + 15 = 35 minutes

The trip takes \(35\) minutes.

Example 4 — Find the finish time
Choir starts at \(5\!:\!50\) and lasts \(40\) minutes. What time does it finish?
Solution
\(5\!:\!50 \to 6\!:\!00\)\(=\)\(10\text{ min used}\)
\(40 - 10\)\(=\)\(30\text{ min left}\)
\(6\!:\!00 + 30\text{ min}\)\(=\)\(6\!:\!30\)
6:00+30 min =6:30

Choir finishes at \(6\!:\!30\).

Example 5 — Find the start time
A film finishes at \(3\!:\!20\). It ran for \(45\) minutes. What time did it start?
Solution
\(3\!:\!20 \to 3\!:\!00\)\(=\)\(20\text{ min back}\)
\(45 - 20\)\(=\)\(25\text{ min still to go}\)
\(3\!:\!00 - 25\text{ min}\)\(=\)\(2\!:\!35\)
3:20 - 45 minutes = 2:35

Count back. The film started at \(2\!:\!35\).

Example 6 — Hours and minutes
A car trip starts at \(9\!:\!30\) am and ends at \(11\!:\!05\) am. How long is the trip, in hours and minutes?
Solution
\(9\!:\!30 \to 10\!:\!00\)\(=\)\(30\text{ min}\)
\(10\!:\!00 \to 11\!:\!00\)\(=\)\(1\text{ h}\)
\(11\!:\!00 \to 11\!:\!05\)\(=\)\(5\text{ min}\)
\(30 + 5\)\(=\)\(35\text{ min}\)
1 hour 35 minutes

The trip takes \(1\) hour \(35\) minutes, which is \(95\) minutes.

Common pitfalls

Time is not base ten. An hour has 60 minutes, not 100. From \(5\!:\!35\) to \(7\!:\!05\) is \(1\) hour \(30\) minutes, not \(2\) hours \(30\) minutes. Count on instead of subtracting the digits.
Forgetting to bridge the hour. When the finish minutes are smaller than the start minutes, count up to the next o'clock first. From \(11\!:\!40\) to \(12\!:\!15\) is \(35\) minutes, not \(75\).
Losing count of the whole hours. After bridging the hour, count each o'clock carefully. It helps to write the o'clock times on the timeline before you jump.

Frequently asked questions

What is elapsed time?

Elapsed time is the amount of time that passes between a start time and a finish time. It is also called the duration, or how long something lasts.

How do you find elapsed time?

Count on from the start time to the finish time along a timeline. First count the minutes up to the next o'clock, then count the whole hours, then count the last minutes. Add the jumps together.

Why can't you just subtract the digits in the times?

Time is not base ten. An hour has 60 minutes, not 100, so subtracting the columns gives the wrong answer. From \(5\!:\!35\) to \(7\!:\!05\) is \(1\) hour and \(30\) minutes, not \(2\) hours \(30\) minutes.

How many minutes are in an hour?

There are \(60\) minutes in \(1\) hour. Half an hour is \(30\) minutes and a quarter of an hour is \(15\) minutes.

What does bridging the hour mean?

Bridging the hour means first counting the minutes up to the next o'clock. From \(11\!:\!40\) you count \(20\) minutes to reach \(12\!:\!00\), then keep counting on from there.

How do you find a finish time when you know how long something lasts?

Start at the start time and count on the duration. If choir starts at \(5\!:\!50\) and lasts \(40\) minutes, count \(10\) minutes to \(6\!:\!00\) and \(30\) minutes more to finish at \(6\!:\!30\).

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