Elapsed time is how much time passes between two times. If an activity starts at 10:00 and ends at 10:45, the elapsed time is 45 minutes. Count on or calculate the difference.
Use timelines and number lines. Count on from start to end time by 5s and 1s.
Confusing start and end times; incorrectly adding instead of subtracting; times crossing the hour.
You already know how to tell time to the minute — reading a clock face and saying "it's 10:35." Elapsed time asks a different question: not "what time is it now?" but "how long did something take?" If a movie starts at 1:15 and ends at 3:00, elapsed time asks: how many minutes or hours passed between those two points?
The trap students fall into is treating this like ordinary subtraction — subtracting 1:15 from 3:00. But time doesn't work like regular numbers, because hours have 60 minutes, not 100. Trying to do "3:00 minus 1:15" as if it were "300 minus 115" gives the wrong answer. The reliable approach is to use a timeline (or open number line): mark the start time, mark the end time, and figure out the jumps in between.
Here is a reliable strategy: jump to the next clean hour, then count the remaining minutes. Starting at 1:15, count up to 2:00 — that's 45 minutes. Then from 2:00 to 3:00 is exactly 60 minutes. Total: 45 + 60 = 105 minutes, or 1 hour and 45 minutes. This approach works because it breaks the problem into chunks you can count confidently, rather than trying to do arithmetic on hours and minutes simultaneously.
The same strategy works in reverse: if you know the start time and how long something lasts, you add the elapsed time to find the end time. Start at 9:40, activity lasts 35 minutes — jump from 9:40 to 10:00 (20 minutes), then add the remaining 15 minutes to get 10:15. The key habit to build is drawing the number line instead of doing the calculation in your head. Visualizing time as a line, where you move right as time passes, turns an abstract problem into something you can see and count.