12-Hour and 24-Hour Time

What you will learn

• Read and write times in 12-hour and 24-hour form.
• Convert between them.
• Understand midnight, noon, am and pm.

Before you start: you should be able to tell the time to five minutes. See Quarter Past, Quarter To and Five-Minute Times.

The idea

A clock face shows $12$ hours, so a day has two trips round it. The 12-hour clock uses am (morning, from midnight to noon) and pm (afternoon and evening, from noon to midnight) to tell them apart. The 24-hour clock counts straight through from $00{:}00$ to $23{:}59$, so it needs no am or pm.

Converting

• Morning (am): the hour stays the same, with a leading zero if you like. $7{:}30$ am is $07{:}30$.
• Afternoon and evening (pm): add $12$ to the hour. $3{:}45$ pm is $15{:}45$.
• Special cases: $12{:}00$ midnight is $00{:}00$. $12{:}00$ midday stays $12{:}00$.

From 24-hour back to 12-hour

If the hour is $13$ or more, subtract $12$ and write pm. $18{:}05$ is $6{:}05$ pm.

Worked examples

Example 1: pm to 24-hour

Write $8{:}20$ pm in 24-hour time.

1. It is pm, so add $12$ to the hour: $8 + 12 = 20$.
2. Keep the minutes.

Answer: $20{:}20$

Example 2: 24-hour to 12-hour

Write $17{:}10$ in 12-hour time.

1. The hour is more than $12$, so subtract $12$: $17 - 12 = 5$.
2. Add pm.

Answer: $5{:}10$ pm

Example 3: Reading a timetable

A bus leaves at $15{:}35$. A friend says “that is twenty-five to four”. Is that right?

1. $15{:}35$ is $3{:}35$ pm.
2. $3{:}35$ is $25$ minutes before $4$ o clock.

Answer: Yes, it is twenty-five to four in the afternoon.

Watch out: $12{:}30$ pm is $12{:}30$ in 24-hour time, but $12{:}30$ am is $00{:}30$.

Why it matters

Train and bus timetables, flight tickets, hospital letters and many digital clocks use 24-hour time. Reading it correctly stops you missing a departure.