Elapsed time calculator

How to calculate elapsed time

Elapsed time is how much time passes between a start time and an end time. The method most schools teach is to count up on an open number line, in friendly jumps:

1. Jump to the next hour. From 8:45 to 9:00 is 15 minutes.
2. Jump by whole hours. From 9:00 to 11:00 is 2 hours.
3. Jump the minutes left. From 11:00 to 11:20 is 20 minutes.
4. Add the jumps. 15 minutes + 2 hours + 20 minutes = 2 hours 35 minutes.

That's exactly what the calculator draws. The jumps aren't to scale (a classroom number line isn't either), but longer jumps are wider so children can see that 2 hours is more than 15 minutes.

The Common Core standard for 3rd grade (3.MD.A.1) asks children to measure time intervals in minutes and solve word problems "by representing the problem on a number line diagram." In 4th grade (4.MD.A.2), they solve problems with intervals of time using all four operations, which includes converting: 2 hours 35 minutes is 155 minutes.

Elapsed time strategies, compared

Children don't all think about time the same way. Teachers usually show more than one strategy and let each child use the one that clicks.

The number line, step by step

Draw a line and write the start time at the left. Ask, "How far to the next o'clock?" and draw that jump. Then jump by hours until the next jump would go past the end time, and finish with the leftover minutes. Write each jump above the line. The calculator does all of this so children can check their own drawing.

Counting on a clock

For short times, a clock works well. Set the start time on the interactive clock, then press +5 min and count by 5s until you reach the end time. Children see the minute hand travel and the hour hand creep, which also reinforces telling time.

Subtracting (and the 60-minute trap)

Subtracting works once children know that an hour has 60 minutes. For 11:20 minus 8:45, 20 minutes is less than 45, so borrow an hour from the 11: 10 hours 80 minutes minus 8 hours 45 minutes is 2 hours 35 minutes. The common mistake is to regroup like ordinary numbers, borrowing 100 instead of 60. Counting up avoids the trap entirely.

Finding an end time or a start time

The same jumps work in reverse. To find an end time, start at the start time and add the duration in jumps: 2:40 p.m. plus 1 hour 35 minutes is 3:40 (one hour), then 20 minutes to 4:00, then 15 more minutes to 4:15 p.m. To find a start time, count back from the end time. Pick Start + duration or End − duration in the calculator to see both on the number line.

Crossing noon and midnight

Problems get harder when the time crosses 12:00, because a.m. turns into p.m. (at noon) or p.m. turns into a.m. on the next day (at midnight). The fix is to make 12:00 one of the jumps.

When the end time is earlier on the clock than the start time, the calculator counts into the next day and says so. The number line marks noon and midnight by name.

Elapsed time practice

The Practice tab has word problems at four levels: under an hour, hours and minutes, to the minute, and across noon or midnight. Each round mixes three kinds of question, like the ones on tests:

• How long? "Maya starts reading at 3:40 p.m. and finishes at 4:15 p.m. How long is that?"
• What time does it end? "Leo practices piano for 45 minutes, starting at 4:30 p.m."
• What time did it start? "Ava finishes baking at 10:05 a.m. It took 1 hour 20 minutes."

Answers can be typed the way children write them: "1 h 25 min", "1 hour 25 minutes", "85 minutes" or "1:25" for how long, and "4:10" or "4:10 p.m." for times. After each answer, the number line shows how to solve it, so a wrong answer becomes a worked example.

For printed practice, the elapsed time worksheets and the 3rd grade elapsed time worksheets come with answer keys. The number line generator makes blank lines for children to draw their own jumps.

Elapsed time and telling time

Elapsed time shows up everywhere once you look: how long until the bus, how long the movie lasts, how long the cookies have been baking. Ask those questions out loud during the day and let your child work them out.

Elapsed time builds on telling time. A child who still mixes up the hour hand will struggle to find a start time. If the number line seems confusing, spend a few days on the telling time game at the 5 minutes or 1 minute level first. Use a start time and a duration that the learner can explain before introducing more complex examples.

Choosing an activity

Grade bands describe possible curriculum connections, not an assessment of an individual learner. Start with an example the learner can explain, then change one feature at a time. The activity suggestions here are editorial teaching ideas, not results from a classroom trial of this software. Use the model alongside written work and physical materials when useful. The linked standards describe learning objectives; they do not endorse Edulist or require this particular tool.