Elapsed time, and all time concepts, are tricky to teach. Students seem to either “get it” or not, and there doesn’t seem to be a correlation between proficiency at other mathematical skills and the ability to calculate time. Why? For one, time is more abstract. There isn’t an algorithm to turn to in a pinch. Even adding and subtracting time presents problems when going over an hour span. Another reason might be that students have limited clock experiences. I know that my students rarely have clocks in the home and not many people wear watches anymore with the advent of cell phone clocks. Finally, teachers might be to blame. How often do you tell your students, “Three minutes” only to have five or ten go by before you realize it? No matter the reason, elapsed time can be especially difficult to master. Here are some tips and favorite strategies for teaching elapsed time.
Hold students accountable for their time. If you say that they have five minutes to finish a task, set a timer and stick to it. Students have to own a sense of time for elapsed time to make any sense. Adults, both parents and teachers, tell kids, “Just a minute,” all the time. How often do we actually mean 60 seconds? Parents wake them up; teachers and bells tell them when class starts and ends. We say, “Lunch is in about a half-hour,” when what we mean is 22 minutes. Start engraining time in your day, every day, and students will start to internalize it themselves.
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One of the eight mathematical practice standards in Common Core encourages the use of real-life situations. It turns out that real life and elapsed time go well together. This is a good thing, say Kamii and Russell in their Journal for Research in Mathematics article entitled “Elapsed Time: Why Is It So Difficult to Teach?” Their research suggests, “Students must be encouraged to think about durations in daily living and do their own thinking . . . ” in order to be successful at elapsed time.
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Don’t give your students the subtraction strategy for time! DON’T! What will happen when they have to go over an hour? What about when they have to convert hours to minutes? The problem with subtraction and the “algorithm” for time is that students who don’t truly grasp time are lost. They can’t transfer their knowledge from one kind of time problem to the next. Only teach students strategies that can be applied to other problems. If the strategy doesn’t work repeatedly, don’t do it!
I know, I just said not to teach algorithms, but there are some strategies that work. Strategies work, not algorithms. Students still have to understand and apply their knowledge in order to work the problem, but these two methods make it easier for students to be successful.
Using a T-chart to find elapsed time is an easy strategy for students. They can lay out their starting time and ending time in an organized way. It gives them an entry point for a problem, but when students are given an ending time and asked to move backwards a specific amount of time, the computation gets tricky.
This is, by far, my favorite strategy for teaching elapsed time. I don’t know the creator of this strategy, but I would love to shake their hand. Mountains are an hour, hills are five-minute increments, and rocks are one-minute increments. Using a time line, students make a visual representation of their thinking. Not only is it a fast way to find solutions, but this strategy also lets you see exactly where students go wrong when they are having problems. There is no guessing what they meant, and they have to fully understand time. Finally, this strategy works for the student who can tell time fluently, and also for remedial students who cannot count more than five minutes at a time.
To practice this strategy, I give students one question from a page of problems. Pairs of students draw their work on a piece of chart paper and then we hang them around the room. Each chart gets a letter assigned to it. Students then get the full assignment and walk around matching their questions with the work displayed by writing the letter of the answer chart next to the problem. It gives them practice while showing the common errors and how mistakes affect the problems.
What other strategies do you use for elapsed time? I’ve heard of teachers making giant “walkable” clocks on their floor and of variations on the time line. What has worked in your classroom?