Time and Clocks

What you will learn

• Read analogue and digital clocks and tell the time to the minute.
• Switch between 12-hour and 24-hour time.
• Convert units of time and work out how long something takes.

Before you start: you should be able to count in $5$s and know what a calendar is. See Skip Counting in 2s, 5s and 10s and Calendars, Days and Months.

The idea

Most of our measurements are based on tens, but time is not. There are $60$ seconds in a minute, $60$ minutes in an hour and $24$ hours in a day. These numbers come from ancient Egypt and Babylon, where people counted in twelves and sixties. Sixty divides neatly into halves, thirds, quarters, fifths and sixths, which makes it handy for sharing out time.

Reading an analogue clock

An analogue clock has a short hour hand and a long minute hand. The face has $12$ numbers, and the minute hand takes one hour to go all the way round.

• When the minute hand points straight up at $12$, it is o’clock.
• At the $6$, it is half past (30 minutes).
• At the $3$, it is quarter past; at the $9$, it is quarter to.
• Each number on the face is worth $5$ minutes to the minute hand.

12-hour and 24-hour time

The 12-hour clock uses am (before midday) and pm (after midday). The 24-hour clock, used on timetables and phones, counts straight through from $00\!:\!00$ (midnight) to $23\!:\!59$. To convert a pm time, add $12$ to the hours: $7\!:\!15$ pm is $19\!:\!15$.

Worked examples

Example 1: Reading a clock

The hour hand is just past $4$ and the minute hand points at $8$. What time is it?

1. The hour hand has passed $4$ but not reached $5$, so the hour is $4$.
2. The minute hand at $8$ means $8 \times 5 = 40$ minutes past.
3. That is also ”$20$ to $5$”, because $60 - 40 = 20$.

Answer: $4\!:\!40$ (twenty to five).

Example 2: How long is the journey?

A train leaves at $09\!:\!47$ and arrives at $12\!:\!15$. How long does the journey take?

1. Count up to the next whole hour: $09\!:\!47 \to 10\!:\!00$ is $13$ minutes.
2. Then whole hours: $10\!:\!00 \to 12\!:\!00$ is $2$ hours.
3. Then the remaining minutes: $12\!:\!00 \to 12\!:\!15$ is $15$ minutes.
4. Add: $2$ hours and $13 + 15 = 28$ minutes.

Answer: $2$ hours $28$ minutes. A calculator subtraction ($12.15 - 9.47$) would give the wrong answer, because time is not decimal.

Example 3: Decimal hours

A walk takes $2.4$ hours. How long is that in hours and minutes?

1. The whole part is $2$ hours.
2. Convert the decimal part: $0.4 \times 60 = 24$ minutes.

Answer: $2$ hours $24$ minutes, not $2$ hours $40$ minutes.

Count up rather than subtract when working out durations. A number line with jumps to the next hour makes it hard to go wrong.

Why it matters

Time is the measurement we use most: catching buses, cooking, taking medicine at the right intervals, planning a day, timing a race. Reading a 24-hour timetable, working out how long until a deadline, and converting between time zones when calling family abroad are all practical skills that rely on this lesson.