Time
Time measures the duration between events using standardized units where 60 seconds equal 1 minute, 60 minutes equal 1 hour, and 24 hours equal 1 day. Converting between time units requires multiplying or dividing by these base numbers, while calculating elapsed time involves counting forward from a starting point to an ending point. The 24-hour clock system expresses afternoon and evening hours as numbers 13 through 23, eliminating AM/PM confusion.
Why it matters
Time calculations appear throughout daily life, from cooking recipes that require precise timing to work schedules spanning multiple hours. Students encounter elapsed time problems in CCSS.3.MD when determining how long events last or when activities will end. Converting time units becomes essential in science classes where experiments might run for 150 minutes (2 hours 30 minutes) or in sports where a 90-minute soccer match translates to 1 hour 30 minutes. Understanding 24-hour format proves crucial for reading train schedules, military operations, and international communication where 15:30 means 3:30 PM. These skills build toward more complex time zone calculations and rate problems in higher mathematics.
How to solve time
Time
- 60 seconds = 1 minute; 60 minutes = 1 hour; 24 hours = 1 day.
- To convert hours to minutes: multiply by 60.
- Elapsed time: count forward from start to end.
- 24-hour clock: add 12 to pm hours (e.g. 3 pm = 15:00).
Example: 2 h 30 min = 2 × 60 + 30 = 150 minutes.
Worked examples
How many days in 1 week?
Answer: 7
- Recall the time fact → 7 — Count on your fingers: Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday. That's 7 days in every week.
- State the answer clearly → There are 7 days in 1 week — The answer is 7. This is a basic time fact worth memorising, just like knowing there are 10 fingers on your hands.
School starts at 8:00 PM. The school day lasts 1 hour. What time does school end?
Answer: 9:00 PM
- Read the starting time → Start: 8:00 PM — We begin at 8:00 PM. Think of where the hour hand is pointing on a clock.
- Count 1 hour forward → 20 + 1 = 21 — Add 1 to the hour: 20 + 1 = 21.
- Write the final time → 9:00 PM — The answer is 9:00 PM. On a 24-hour clock, that's 21:00.
A football match starts at 2:00 PM and lasts 2 hours 45 minutes. When does it end?
Answer: 4:45 PM
- Read the starting time → Start: 2:00 PM — The event begins at 2:00 PM. Write down the start hour (14) and start minutes (0) separately.
- Add the hours first → 14:00 + 2h = 16:00 — Adding hours is easy -- just move the hour hand forward by 2. We go from hour 14 to hour 16.
- Add the minutes → 0 + 45 = 45 min — Add 45 minutes to 0: 0 + 45 = 45. This is less than 60, so no carrying needed.
- Combine into final time → 4:45 PM — The event ends at 4:45 PM. Think of it like this: 2:00 PM plus 2 hours 45 minutes lands you at 4:45 PM.
Common mistakes
- Adding times incorrectly by treating minutes like base-10 numbers, such as calculating 2:45 PM + 30 minutes as 2:75 PM instead of 3:15 PM.
- Converting hours to minutes by adding instead of multiplying, writing 3 hours = 63 minutes (3 + 60) instead of 180 minutes (3 × 60).
- Forgetting to carry over when minutes exceed 59, such as writing 1:40 + 25 minutes = 1:65 instead of 2:05.