Lesson 81 — Time Zones and Duration
Strand: Measurement | Descriptor: AC9M8M04 | Duration: 45 minutes
Learning Intentions
- To understand how the world is divided into time zones relative to Coordinated Universal Time (UTC).
- To convert times between time zones using UTC offsets, including Australian zones and daylight saving.
- To calculate elapsed time (duration) for events and journeys that cross one or more time zones.
Success Criteria
I can:
- Explain what UTC is and describe a time zone offset such as UTC+10 or UTC−5.
- State the standard time zones used across Australia (AWST, ACST, AEST) and their daylight-saving equivalents.
- Convert a given 24-hour time in one time zone to the equivalent time in another zone.
- Calculate the duration of a journey or event that crosses one or more time zones.
Warmup
(5 minutes — mini whiteboards, rapid-fire)
Display a simple world map with three pins: Perth, Sydney, London.
- It is 3:00 pm in Perth. Do you think it is earlier or later than 3:00 pm in Sydney? Why?
- When you fly from Australia to the UK, why does your phone’s clock jump by several hours the moment you land?
- Sydney is east of Perth. Which city’s sun rises first — and does that mean its clock reading is ahead or behind?
Teacher note: Question 3 is the hook — many students confuse “sees the sun first” with “behind in time”. Do not resolve it yet; return to it explicitly during Activity 1.
Activities
Activity 1 — Explicit Instruction: UTC and Time Zone Offsets (12 min)
Explain that every time zone is defined by its offset from Coordinated Universal Time (UTC), written as UTC+n or UTC−n. Zones to the east of Greenwich (UK) have positive offsets (their clocks are ahead); zones to the west have negative offsets (their clocks are behind).
Display key Australian and world zones (standard time, no daylight saving):
| City | Zone | UTC Offset |
|---|---|---|
| Perth | AWST | UTC+8 |
| Darwin | ACST | UTC+9:30 |
| Brisbane | AEST | UTC+10 |
| Sydney / Melbourne / Hobart | AEST | UTC+10 |
| Tokyo | JST | UTC+9 |
| London | GMT | UTC+0 |
| Los Angeles | PST | UTC−8 |
| Auckland | NZST | UTC+12 |
I do: “It is 16:00 in Sydney (UTC+10). What time is it in Tokyo (UTC+9)?” Since Tokyo’s offset is 1 hour less than Sydney’s, Tokyo’s clock is 1 hour behind.
We do: Together, find the time in London when it is 16:00 in Sydney.
You do: It is 09:00 in Perth. Find the time in (a) Sydney (b) Tokyo (c) London (d) Los Angeles.
Now address daylight saving directly: from early October to early April, NSW, Vic, SA, Tas and the ACT put clocks forward 1 hour (AEDT = UTC+11, ACDT = UTC+10:30). Qld, WA and the NT do not observe daylight saving. This is why, in January, Brisbane and Sydney can show different times even though both are normally described as “Eastern”.
Activity 2 — Explicit Instruction: Duration across Time Zones (12 min)
Explain the key strategy: when an event crosses time zones, convert all times to a single common reference (usually UTC) before subtracting.
I do: A flight leaves Perth (UTC+8) at 21:30 and lands in Sydney (UTC+11, daylight saving) at 07:05 the next day, local time. Find the actual flight duration.
Both times now refer to the same UTC day, so subtract directly:
We do: A call is made from London (UTC+0) at 20:00. What is the local time in Auckland (UTC+12) at that instant?
You do: A cargo ship’s log shows departure from Los Angeles (UTC−8) at 14:00 on Monday, arriving in Tokyo (UTC+9) at 18:20 local time on Wednesday. Find the voyage duration.
(Answer: convert both to UTC — depart 22:00 Monday UTC, arrive 09:20 Wednesday UTC; duration = 1 day 11 h 20 min = 35 h 20 min.)
Activity 3 — Applied Task: Scheduling a Global Video Call (10 min)
Pairs, then whole-class share.
A company has offices in Perth (UTC+8), London (UTC+0) and Los Angeles (UTC−8, no daylight saving this month). They need a video call where every office is between 9:00 am and 5:00 pm local time. Find every possible UTC time the call could start, or explain why no such time exists.
Socratic scaffolding:
| Prompt | Purpose |
|---|---|
| Understand: what exactly is being asked? | Not “what time is convenient in Perth” — a single UTC instant that satisfies all three local constraints simultaneously. |
| What do you know? | Each office’s working hours, expressed as a UTC range using its offset. |
| Devise a plan | Convert each office’s 9 am–5 pm window into UTC, then find the overlap (intersection) of all three ranges. |
| Carry out the plan | Perth window in UTC: |
| Look back — does an overlap exist? | Perth’s window (01:00–09:00 UTC) and London’s window (09:00–17:00 UTC) touch only at the single instant 09:00 UTC — but LA’s window doesn’t begin until 17:00 UTC. There is no common overlap. |
| Look back — is the answer sensible? | Perth and LA sit almost on opposite sides of the clock — a genuine real-world scheduling problem. Ask: what compromise could the company make? (e.g. one office joins outside its normal 9–5) |
Checks for Understanding
(6 minutes — exit ticket, collected)
- Write the UTC offset for AEDT (Australian Eastern Daylight Time).
- It is 14:20 in Brisbane (UTC+10). What time is it in Perth (UTC+8)?
- A live broadcast starts at 10:00 in London (UTC+0). What local time do viewers in Sydney (UTC+11, daylight saving) see it?
- A flight departs Tokyo (UTC+9) at 23:40 and arrives in Perth (UTC+8) at 06:15 local time the next day. Find the flight duration.
Answers: 1. UTC+11; 2. 12:20; 3. 21:00 the same day; 4. Convert to UTC: depart
Common Misconceptions
| Misconception | How to pre-empt it |
|---|---|
| A city further east always shows a later clock time than one further west, by any amount. | Distinguish “sees sunrise first” (always true, east) from “clock reads a larger number” (depends on the defined offset, not geography alone). |
| Daylight saving applies uniformly across all of Australia. | Explicitly list which states/territories observe it (NSW, Vic, SA, Tas, ACT) and which do not (Qld, WA, NT). |
| Local times from different zones can be subtracted directly without converting to a common reference first. | Model Activity 2 explicitly: always convert to UTC (or one agreed zone) before subtracting. |
| Crossing into “the next day” only happens when crossing the International Date Line. | Clarify that in these problems, “next day” arrivals come from elapsed hours exceeding 24 in the common reference, not from the Date Line — a separate, later topic. |
| UTC offsets are always whole numbers of hours. | Show the counterexample of ACST (UTC+9:30) to demonstrate half-hour offsets exist. |
Enrichment — Competition-Style Problems
E1 (Kangaroo style). Three friends live in cities with UTC offsets
Answer
Convert
Friend at UTC−3:
Sum of the hour values:
E2 (AMC Junior style). A radio program airs live at 20:00 UTC. In a city 5 time zones ahead of UTC, a rebroadcast airs exactly 3 hours after the live broadcast reaches that city. At what local time (in that city) does the rebroadcast air?
Answer
Local live time
E3 (Challenge). A ship’s captain, in a time zone of UTC+3, radios a second ship at the moment her clock reads 12:00 noon. The second ship’s crew report their clock reads 08:00 the same day. What is the UTC offset of the second ship, and by how many hours do the two clocks differ?
Answer
Convert the captain’s time to UTC:
The second ship reads
The clocks differ by
Homework
- It is 15:40 in Sydney (UTC+10). Find the time in (a) Perth (UTC+8) (b) London (UTC+0) (c) Tokyo (UTC+9).
- State whether each location observes daylight saving: (a) Adelaide (b) Brisbane (c) Hobart (d) Darwin.
- A radio interview is recorded in Los Angeles (UTC−8) at 09:00 and broadcast live in Melbourne, which is on AEDT (UTC+11) at the time. Find the broadcast’s local start time in Melbourne, stating whether it is the same day or the next.
- A cruise departs Auckland (UTC+12) at 18:00 and arrives in Sydney (UTC+11, daylight saving) at 07:30 local time, two days later. Find the voyage’s duration in hours and minutes.
- Reasoning. Explain, using an example, why two Australian cities that are normally in the same time zone (both AEST) can show different clock times for part of the year.
- Challenge. A satellite passes over three ground stations with UTC offsets
, and . At the instant it is over the second station, its clock reads 14:00. Find the clock readings (24-hour time) at the other two stations at that same instant, and confirm the differences between all three readings match the differences between their offsets.
Answers: 1. (Sydney in UTC: