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Elapsed time measures how much time passes from a start time to an end time. Students count forward by hours and half-hours to find how much time has passed.
You already know how to read a clock and tell time to the half-hour. You know that 3:00 means the short hand points to 3 and the long hand points to 12, and that 3:30 means it is halfway to 4. Elapsed time is a new kind of question: not "what time is it?" but "how long did something take?" These are different questions, and it helps to see why.
Imagine you start eating lunch at 12:00 and finish at 12:30. You did not eat the number 12 or the number 30 — you ate for 30 minutes. Elapsed time is always a duration, measured in hours and minutes, not a time on the clock. To find it, you count forward from the start time to the end time: 12:00 → 12:30 is one half-hour jump, so 30 minutes passed.
A clock face is actually a very helpful tool for counting elapsed time. Picture the long hand at 12 for a start time of 1:00. If you want to find how much time passes until 3:00, you count: from 1:00 to 2:00 is one hour, from 2:00 to 3:00 is another hour — two hours total. Each time the short hand moves from one number to the next, one hour passes. Each time the long hand moves from 12 to 6 (or 6 to 12), a half-hour passes. Counting these jumps gives you the elapsed time.
Try it with a story: you start playing at 2:00 and stop at 4:30. Count forward — 2:00 to 3:00 is one hour, 3:00 to 4:00 is another hour, 4:00 to 4:30 is a half-hour. That is two and a half hours of playtime. The key habit is always to start at the earlier time and count forward to the later time in steps you know well — whole hours first, then half-hours.