Elapsed Time

Elapsed time can be a challenge for kiddos because they like to try to find the difference between two times by lining them up vertically and subtracting for the difference. In a whole-group discussion, we talk about why this won't always work. Our number system is base-10, which means when we reach 10 in a place, we must move it over to the place to the left. For example, when we get 10 ones, we turn it into 1 ten. We also borrow in groups of ten. Time does not follow these rules, so we cannot always successfully borrow or carry vertically.