How to teach elapsed time

Grade 2 to Grade 5

Elapsed time is how long something lasts: the gap between a start time and an end time. It is harder than reading a clock because time is not base ten, there are 60 minutes in an hour, so students cannot just subtract the numbers. Counting on in friendly jumps is the reliable method.

How to teach it

1. Make sure telling time is secure first, since elapsed time depends on reading both the start and the end.
2. Count on rather than subtract. From 2:15 to 3:45, jump 45 minutes to 3:00, then 45 minutes to 3:45, giving 1 hour 30 minutes.
3. Use a number line marked with times, so each jump to the next whole hour is visible.
4. Teach the hour boundary carefully: you cannot borrow ten, an hour is 60 minutes.
5. Progress to problems that cross the hour, then across midday and midnight.

Worked example

How long from 2:15 to 3:45?

   2:15  --> 3:00   is 45 min
   3:00  --> 3:45   is 45 min
   total: 45 + 45 = 90 min = 1 h 30 min

Common mistakes

• Subtracting the times like base-ten numbers (3:45 - 2:15 done as 145 minus 215).
• Forgetting there are 60 minutes in an hour, not 100.
• Losing an hour when the times cross midday or midnight.
• Mixing up the start and end times.