Grades 3–5 · Mathematics
How do we count elapsed time across an hour change?
Elapsed time is the duration between a start and a finish. A clock label describes a moment, while a duration describes how long an interval lasts. The minute part of a clock label runs from zero through fifty-nine and then restarts, but elapsed time does not restart at the hour.
In the diagramElapsed time continues across the hourStart 1:50 · duration 25 minutes1:401:502:002:102:202:3010 min15 min10 + 15 = 25 minutes; end 2:15Clock labels restart at :00; elapsed minutes do not.
The initial interval begins at 1:50 and lasts twenty-five minutes. Ten minutes take the time line to 2:00. Fifteen minutes remain, ending at 2:15. The two connected parts, ten and fifteen minutes, add to the complete twenty-five-minute duration. They are parts of one interval rather than two separate trips.
The time line uses equal spacing for equal numbers of minutes. Crossing the 2:00 mark therefore introduces no gap or extra jump in time. Splitting at a whole hour makes the base-sixty clock labels easier to follow while preserving the same start, finish and total elapsed duration.
About this explanation
Starting knowledge: Read clock times in minutes and add small durations while recognizing that sixty minutes make an hour.
Sources
- Common Core State Standards Initiative — Grade 3: Measuring time intervals in minutes