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Primary - Stage 3 (Year 5 & 6) Stage 3 (Year 5 & 6) Time

Convert 12 to 24-hour

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Theory

To go to 24-hour time, keep a morning hour and add 12 to a pm hour. To go back, subtract 12 from any hour of \(13\) or more and label it pm. The minutes never change.

The 12-hour clock runs \(1\) to \(12\) twice a day, with am for morning and pm for afternoon and evening. The 24-hour clock runs 00 to 23 once through the day.

To go to 24-hour time, keep a morning hour the same and add \(12\) to an afternoon or evening hour.

To go to 12-hour time, subtract \(12\) from any hour of \(13\) or more and write pm; keep smaller hours and write am. The minutes never change.

Add 12 for pm, subtract 12 to read it back. \(9{:}15\) pm is \(21{:}15\); \(14{:}35\) is \(2{:}35\) pm.
Converting between 12-hour and 24-hour time A morning time keeps its hour; a pm time gains 12. Going back, an hour of 13 or more loses 12 and becomes pm. 12-hour 24-hour 9:15 am9:15 pm2:35 pm 09:1521:1514:35 same hour +12 −12
A morning hour is copied across; a pm hour gains \(12\). Going back, an hour of \(13\) or more loses \(12\) and becomes pm.

Every afternoon and evening hour is exactly \(12\) more on the 24-hour clock.

12-hour24-hour
12 midnight00
9 am09
12 midday12
1 pm13
6 pm18
11 pm23

Converting a time

  1. 12-hour to 24-hour, am: keep the hour, write two digits.
  2. 12-hour to 24-hour, pm: add \(12\) to the hour.
  3. 24-hour to 12-hour, hour \(\geq 13\): subtract \(12\) and write pm.
  4. 24-hour to 12-hour, hour \(\leq 12\): keep the hour and write am. Keep the minutes the same throughout.
Example 1 — Morning to 24-hour
Write \(9{:}15\) am in 24-hour time.
Solution

The hour does not change before midday.

The time is 09:15.

Example 2 — Evening to 24-hour
Write \(9{:}15\) pm in 24-hour time.
Solution
hour\(=\)\(9 + 12 = 21\)

The time is 21:15.

Example 3 — 24-hour to afternoon
Write \(14{:}35\) as a 12-hour time.
Solution
hour\(=\)\(14 - 12 = 2\)

The time is 2:35 pm.

Example 4 — 24-hour to morning
Write \(07{:}40\) as a 12-hour time.
Solution

The hour is under \(12\), so it stays. Drop the leading zero and label am.

The time is 7:40 am.

Common pitfalls

Adding 12 to the minutes. Add \(12\) to the hour only. \(9{:}15\) pm is \(21{:}15\), not \(9{:}27\).
Putting a 1 in front instead of adding. \(9\) pm is \(21\), not \(19\). You add \(12\) to the hour.
Leaving off am or pm on a 12-hour time. On its own \(2{:}35\) could be either half of the day, so it needs am or pm; never write \(14{:}35\) pm.

Frequently asked questions

How do I change a pm time to 24-hour time?

Add \(12\) to the hour and keep the minutes. \(9{:}15\) pm becomes \(9 + 12 = 21\), so \(21{:}15\).

How do I change an am time to 24-hour time?

Keep the hour the same and write it with two digits. \(9{:}15\) am becomes \(09{:}15\).

How do I turn a 24-hour time back into a 12-hour time?

If the hour is \(13\) or more, subtract \(12\) and write pm. \(14{:}35\) becomes \(2{:}35\) pm. Smaller hours stay and take am.

Why is 9 pm not 19?

Because you add \(12\), not a \(1\). \(9 + 12 = 21\), so \(9\) pm is \(21\).

Do the minutes ever change?

No. Converting only changes the hour. The minutes are copied across unchanged.

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