Elapsed Time and Base-Sixty Regrouping
Time arithmetic regroups at sixty (seconds, minutes) and at twelve/twenty-four (hours, AM/PM), with days and weekdays cycling on top. Problems find elapsed time between clock readings, future dates, and the error of a fast or slow clock. It is place-value regrouping in a mixed-base system.
grade 2–3 MDOA Draw a DiagramWork Backwards
Progression (10)
2-2 1. Compare hours first, then minutes 2.MD.C.7
2-2 2. Find a future date using days per month 1.MD.B.3
2-2 3. Elapsed time between two clock times 3.MD.A.12.MD.C.7
2-2 4. Reason about a fast or slow clock 3.MD.A.1
2-2 5. Reach the next hour, then add the rest 2.MD.C.73.MD.A.1
2-2 6. Read a mirrored clock, then find elapsed time 2.MD.C.73.MD.A.1
2-2 7. Read the hour when the hand is between 2.MD.C.7
3-1 8. AM and PM make a 24-hour day 3.MD.A.1
3-1 9. A drifting clock accumulates daily error 3.MD.A.13.OA.A.3
3-1 10. Time regroups in base sixty 3.MD.A.1