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Time

2.0 Measurement · 2.5 Time

Syllabus tag: Kenya CBC | Grade 6 Mathematics | Strand 2.0 Measurement | Sub-Strand 2.5 Time (10 Lessons)

Lesson objectives

By the end of this topic, you will be able to:

  • Identify and write time in a.m. and p.m.
  • Relate time in a.m. and p.m. to the 24-hour clock system
  • Convert time from the 12-hour to the 24-hour system, and back
  • Interpret travel timetables
  • Appreciate the use of time in both 12h and 24h systems

Time

A day has 24 hours. We can write the time in two ways, and this topic changes between them.

a) a.m. and p.m.

a.m. means before midday. It covers midnight to just before noon.

p.m. means after midday. It covers noon to just before midnight.

So 7.00 a.m. is early morning and 7.00 p.m. is evening.

Two special ones catch people out. Midnight is 12 a.m. and midday is 12 p.m.

b) The 24-hour clock

The 24-hour clock counts straight through from 0000 to 2359. It needs no a.m. or p.m., because every time is different.

12-hour and 24-hour 12-hour and 24-hour 12-hour 24-hour 6.00 a.m. 0600 h 12 noon 1200 h 3.00 p.m. 1500 h 9.30 p.m. 2130 h

Always write four digits. Nine in the morning is 0900 h, not 900 h.

c) Converting

Converting to 24-hour Converting to 24-hour morning times = keep the number a.m. stays the same 7.00 a.m. = 0700 h just add a zero in front afternoon times = add 12 p.m. needs this 4.00 p.m. → 4 + 12 = 1600 h the answer

For a morning time, keep the number and make it four digits.

For an afternoon or evening time, add 12 to the hours.

To go back the other way, subtract 12 from any hour above 12, and write p.m.

So 1800 h becomes 6.00 p.m., and 0800 h becomes 8.00 a.m.

d) Reading a timetable

Travel timetables use the 24-hour clock, so there is no confusion between morning and evening.

Read down the column for one journey. Read across the row for one stop.

To find how long a journey takes, subtract the departure time from the arrival time.

A bus leaving at 0930 h and arriving at 1215 h takes 2 hours 45 minutes.

Remember that an hour is 60 minutes, not 100. From 0930 to 1215 is not 2.85 hours.

e) Where this is used

Catching a matatu or a train. Watching a television schedule. Booking a flight. Keeping to a school timetable.

Words to know

  • a.m. (ante meridiem) — the hours from midnight up to (but not including) noon.
  • p.m. (post meridiem) — the hours from noon up to (but not including) midnight.
  • 24-hour clock — a time system running from 00:00 (midnight) to 23:59, with no a.m./p.m. needed.
  • Duration — the length of time between a start and an end time.

:::checkpoint Check yourself

  1. Write 5.00 p.m. in the 24-hour clock.
  2. Write 1400 h in the 12-hour clock.
  3. Is 12 midnight written as a.m. or p.m.?
  4. A bus leaves at 0800 h and arrives at 1130 h. How long is the journey? :::

Bridge to practice

Try the exercises below — converting between 12-hour and 24-hour time, and working out journey durations from timetables.

Check yourselfPractise Time — 10 questions →Next in MathematicsMoney →