Time & time zones

What you will learn

convert between 12- and 24-hour time,
calculate durations across noon and midnight,
use standard time zone offsets (UTC) to find local time elsewhere,
plan travel and meetings that span multiple time zones,
account for daylight saving when it is relevant.

1. 12-hour vs 24-hour time

12-hour time uses a.m./p.m.; 24-hour time uses hours $00$ - $23$ .

12-hour	24-hour
$12$ midnight	$00{:}00$
$6$ a.m.	$06{:}00$
$12$ noon	$12{:}00$
$3{:}30$ p.m.	$15{:}30$
$11{:}45$ p.m.	$23{:}45$

For p.m. hours (1 through 11 p.m.), add $12$ to get the 24-hour form.

2. Duration across hours

Break the time span at each hour boundary (or at noon/midnight).

Worked example 1 Duration across midnight

A night shift starts at $22{:}15$ and ends at $06{:}40$ the next day. How long is the shift?

From $22{:}15$ to $24{:}00$ : $1$ h $45$ min.
From $00{:}00$ to $06{:}40$ : $6$ h $40$ min.
Total: $8$ h $25$ min.

3. Time zones

Time zones are expressed as an offset from UTC (Coordinated Universal Time).

Australian standard time zones (no daylight saving)

AEST - Australian Eastern Standard Time

$\text{UTC} + 10$ . Cities: Melbourne, Sydney, Brisbane, Hobart.

ACST - Australian Central Standard Time

$\text{UTC} + 9{:}30$ . Cities: Adelaide, Darwin.

AWST - Australian Western Standard Time

$\text{UTC} + 8$ . Cities: Perth.

To convert between two zones, subtract the offsets.

Worked example 2 Melbourne to Perth

It is $12{:}00$ noon in Melbourne (AEST, UTC+10). What time is it in Perth (AWST, UTC+8)?

Perth is $2$ hours behind Melbourne. $12{:}00 - 2{:}00 = 10{:}00$ a.m.

Worked example 3 Flight arrival time

A flight leaves Sydney (UTC+10) at $18{:}00$ and takes $8$ hours. It lands in Singapore (UTC+8). What is the local arrival time in Singapore?

Arrival in Sydney time: $18{:}00 + 8 = 02{:}00$ next day.
Convert to Singapore time: subtract $2$ hours → $00{:}00$ (midnight, next day).

4. Daylight saving (when it matters)

Most of south-eastern Australia observes daylight saving (AEDT/ACDT) from early October to early April, shifting the clock forward by $1$ hour. Perth, Brisbane, Darwin do not observe daylight saving.

Worked example 4 DST and AEDT

In January, Melbourne observes AEDT (UTC+11). Perth is still AWST (UTC+8). If it is $8{:}00$ a.m. in Melbourne in January, what time is it in Perth?

Perth is $3$ hours behind. $8{:}00 - 3{:}00 = 5{:}00$ a.m.

Practice: Year 8 core

Fluency

Convert and calculate

Convert $4{:}45$ p.m. to 24-hour time.
Convert $23{:}05$ to 12-hour time.
Duration from $9{:}15$ a.m. to $3{:}40$ p.m.
Duration from $22{:}50$ to $07{:}10$ next day.
Add $4$ h $25$ min to $11{:}20$ a.m.
Subtract $3$ h $40$ min from $02{:}15$ next day.

Fluency

Time zones (standard time)

It is $2{:}00$ p.m. in Sydney (AEST). What time in Perth (AWST)?
It is $9{:}30$ a.m. in Adelaide (ACST). What time in Brisbane (AEST)?
It is $10{:}00$ a.m. in Perth (AWST). What time in Melbourne (AEST)?
It is $12{:}00$ noon in London (UTC+0). What time in Melbourne (AEST)?
It is $6{:}00$ p.m. in New York (UTC $-5$ ). What time in Singapore (UTC+8)?

Reasoning

Explain and spot the mistake

Sam says “Melbourne is always $3$ hours ahead of Perth”. Is that always right? Explain.
Explain why a flight from Melbourne to Singapore sometimes lands on the same calendar day and sometimes the next day.
When travelling west, do you gain or lose time on arrival? Explain.
Write $12{:}00$ midnight as two equally valid 24-hour times. Which do most timetables prefer?

Problem solving

Real contexts

A video conference is scheduled for $9{:}00$ a.m. Melbourne time (AEST, April - no DST). What local time should the Perth participant join?
A flight leaves Singapore at $23{:}00$ local (UTC+8) and takes $7$ h $30$ min to Sydney (AEST, UTC+10). What is the local arrival time in Sydney?
A bus leaves Adelaide at $8{:}15$ a.m. (ACST) and arrives in Melbourne at $5{:}45$ p.m. (AEST). How long was the trip (in local times), accounting for the $30$ -minute zone difference?
A family flies Melbourne to Los Angeles (UTC $-8$ ) on a $14$ -hour flight that leaves at $10{:}00$ a.m. Melbourne AEDT (UTC+11) in January. What is the local arrival time in LA?