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
- Make sure telling time is secure first, since elapsed time depends on reading both the start and the end.
- 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.
- Use a number line marked with times, so each jump to the next whole hour is visible.
- Teach the hour boundary carefully: you cannot borrow ten, an hour is 60 minutes.
- 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.
Frequently asked questions
What is elapsed time?
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 you cannot simply subtract the numbers. Counting on in friendly jumps is the reliable method.
What age or grade is elapsed time taught?
Elapsed time is usually taught from Grade 2 to Grade 5, once telling the time is secure. Students start with whole and half hours, then work across the hour, and later across midday and midnight.
Why can't you just subtract the two times?
Because time is not base ten. From 2:15 to 3:45 you cannot do 3:45 minus 2:15 as if it were 345 minus 215, since an hour is 60 minutes, not 100. Subtracting the numbers directly gives the wrong answer, which is why counting on is safer.
What is the count-on method for elapsed time?
Count on in friendly jumps rather than subtracting. From 2:15 to 3:45, jump 45 minutes to reach 3:00, then 45 minutes to reach 3:45, giving 1 hour 30 minutes. A number line marked with times makes each jump to the next whole hour visible.
What should a child know before learning elapsed time?
Telling the time should be secure first, because elapsed time depends on reading both the start and the end accurately. Confusing the start and end times, or misreading the clock, will derail the calculation before it starts.
Why does my child struggle with elapsed time?
Common causes are subtracting the times like base-ten numbers, forgetting there are 60 minutes in an hour, losing an hour when the times cross midday or midnight, and mixing up the start and end. Counting on along a number line, one jump to the next whole hour at a time, avoids most of these.
How do you work out elapsed time across the hour or midnight?
Jump to the next whole hour first, then count the remaining minutes and hours. When crossing midday or midnight, take extra care not to lose an hour, since 12 and 24 hour boundaries are where mistakes happen. A number line keeps the whole span in view.
Practise with free worksheets
Printable worksheets with answer keys that are never wrong.