Time is one of my favorite topics to teach and elapsed time does not have to be as tricky as it may seem. Many students have to find their own way of working out an elapsed time problem. The abstract way of just working it out by adding and subtracting is too difficult for students who need to really 'see' and visualize the time period in question.
What do students need to learn in order to understand elapsed time? First of all, they need to know that even though an analog clock only shows twelve hours, there are actually twenty-four hours in a day. Elapsed time rulers can be use to demonstrate this. I use the 24 hour ruler (a laminated printable so it's bendable) by curving it into a circle so that the 12:00 a.m. point is overlapped onto the other 12:00 a.m. that is on the other end of the ruler, exactly 24 hours further down. The circle will help them to see the continuous cycle of day and night. Students also need to know that to find elapsed time, they must know the starting time and finishing time. And likewise, if they are already given the elapsed time, they can use it to find the finishing time if given the starting time, and vice versa.
The picture above shows how elapsed time rulers can be used like a number line for students to count on from the starting time to the completion time of an event. The rulers give students a visual picture of the time span and with enough practice, students can visualize time spans in their heads better than they previously could have. This has given my students an easier and faster alternative to solely using their working columns to calculate the answers.
I decided to color-code the elapsed time rulers to match our clocks from Learning Resources by having red stokes for the hours (upper half) and blue strokes for the minutes (lower half). Five minute and fifteen minute increments are also shown in blue. I wanted to have consistency in this because the color-coded Learning Resources Clocks have really made it easier for them to learn about time when I was teaching it and I know that many other teachers out there use these clocks as well.
My elapsed time rulers are available in three different time spans:
- 24 hour rulers
- a.m. 12 hour rulers
- p.m. 12 hour rulers
The set is available here at my TpT collection and teachers can print them on transparent sheets to use with an overhead projector to demonstrate their use. Each ruler is 31.5cm long (that's just over a foot long). To print them out for the students, use 8.5 x 14 (legal) sized paper to accommodate the full length of the rulers. The rulers shown in the preview here are the 12:00 a.m (midnight) to 12:00 p.m. (noon) 12 hour rulers. Check it out to see more pictures!