ACARA v9 CONTENT DESCRIPTION “compare 12- and 24-hour time systems and solve practical problems involving the conversion between them”
Builds on reading clocks and telling the time to the minute. Year 5 compares the two systems used to write the time: the familiar 12-hour clock with am and pm, and the 24-hour clock used on timetables and digital displays. Learning how the two systems line up, converting confidently between them, and solving the everyday problems that involve a timetable or a digital clock makes time something that can be read and used in any form.
Two ways to tell the time
There are two common ways to write the time. The 12-hour system counts the hours from one to twelve twice a day, using am for the morning hours after midnight and pm for the afternoon and evening hours after noon. The 24-hour system counts straight through from zero to twenty-three, so there is no need for am or pm. Nine in the morning is nine am or oh-nine hundred; half past three in the afternoon is three thirty pm or fifteen thirty. The two systems describe the same moments in different words.
Two ways to tell the time
The 12-hour and 24-hour systems describe the same moment.
9:00 am and 09:00 are the same moment written in the two systems.
From 12-hour to 24-hour time
Converting from 12-hour to 24-hour time follows a simple rule. For times in the morning, am, the 24-hour time is the same, except that twelve midnight becomes zero zero. For times in the afternoon and evening, pm, twelve hours are added to the hour, so three thirty pm becomes fifteen thirty and eleven pm becomes twenty-three hundred. The minutes never change. Twelve noon stays as twelve hundred, since it is already twelve. Knowing whether a time is am or pm is the key to writing it correctly in 24-hour form.
From 12-hour to 24-hour time
am stays the same; pm adds twelve hours.
Convert this 12-hour time to 24-hour time.
From 24-hour to 12-hour time
Converting from 24-hour to 12-hour time reverses the rule. Hours from zero to eleven are morning times, am, with zero zero meaning twelve midnight. Hours from twelve to twenty-three are afternoon or evening times, pm, found by subtracting twelve from the hour, so twenty fifteen becomes eight fifteen pm. Twelve hundred is twelve noon. Again the minutes stay the same. Reading a 24-hour time means deciding first whether the hour is below twelve, which makes it am, or twelve and above, which makes it pm.
From 24-hour to 12-hour time
12 to 23 is pm; subtract twelve from the hour.
Convert this 24-hour time to 12-hour time.
Noon, midnight, am and pm
The trickiest moments are noon and midnight. Midnight, the start of the day, is twelve am in the 12-hour system and zero zero in the 24-hour system. Noon, the middle of the day, is twelve pm and twelve hundred. After midnight the morning hours are am, and after noon the afternoon and evening hours are pm. Getting these boundaries right matters, because mixing them up can put an event twelve hours out. Remembering that am runs from midnight to just before noon, and pm from noon to just before midnight, keeps the two halves of the day straight.
Noon, midnight, am and pm
Midnight is 00:00; noon is 12:00.
Choose the correct label for this moment.
Solving problems with time
Many everyday problems involve switching between the two systems. Timetables for buses, trains and flights, and most digital clocks, use 24-hour time, while people often speak and write in 12-hour time. A bus that leaves at fourteen oh five leaves at five past two in the afternoon, and a show that starts at half past seven in the evening is nineteen thirty on the ticket. Solving these problems means converting the time into the form that is needed, then reading it in context. The conversion is the same rule applied to a real situation.
Solving problems with time
Timetables use 24-hour time; convert as needed.
Solve the timetable problem by converting the time.
Telling time with confidence
Telling time with confidence means moving easily between the 12-hour and 24-hour systems. Read whether a time is am or pm, add twelve to afternoon hours to reach 24-hour time, subtract twelve from afternoon 24-hour times to reach 12-hour time, and keep the minutes unchanged. Treat noon and midnight with care, since they are the points where the systems differ most. With these habits a child can read any clock or timetable, convert between the two systems, and solve the practical time problems of everyday life.
Quick self-check
1. The 24-hour clock counts the hours from...
2. In 12-hour time, the hours after noon are labelled...
3. Written in 24-hour time, 3:30 pm is...
4. Written in 12-hour time, 20:15 is...
5. Midnight, the start of the day, in 24-hour time is...