Lesson Guide
Explanation
The Earth turns towards the east, so the sun rises in the east first, and places to the east are ahead. Perth is two hours behind Sydney; Auckland is two hours ahead. Ahead means add the difference, behind means subtract. When it is three in Sydney it is one in Perth and five in Auckland, and Perth to Auckland is the two differences added, four hours. The real zones shift with daylight saving; here the differences are fixed and stated in the question.
Your Turn
Sydney is 2 h ahead of Perth. It is 10 am in Sydney. What time is it in Perth? Check: 8 am.
Watch Out
Behind means EARLIER. Perth's clock shows a smaller time than Sydney's at the same moment.
Explanation
When the difference is large, converting can move the day. Sydney is seventeen hours ahead of Los Angeles. Sydney Monday oh eight hundred, minus seventeen: eight hours back reaches Monday midnight, nine more reach Sunday fifteen hundred. It is still Sunday afternoon in Los Angeles. Going the other way, LA Friday twenty hundred plus seventeen is Saturday thirteen hundred in Sydney. Hop to midnight, then the rest, and write the day name at every step, because losing a day is the usual error.
Your Turn
Sydney Tuesday 0600. What day and time is it in LA (17 h behind)? Check: Monday 1300.
Watch Out
Going back through midnight changes the day to the one BEFORE. Monday 0800 − 17 h is Sunday, not Monday.
Explanation
A flight is two steps done in order. First forget time zones: the plane lands twenty hours after it leaves, so Saturday twenty-three hundred plus twenty is Sunday nineteen hundred, Los Angeles time. Then convert once: Sydney is seventeen hours ahead, and Sunday nineteen hundred plus seventeen is Monday twelve hundred. You left on Saturday night and land on Monday at midday, because the clocks jumped seventeen hours as well as the twenty you spent flying. Converting first gives the same answer, and it must.
Your Turn
LA Saturday 2300 in Sydney time (17 h ahead)? Check: 1 h to Sunday 0000, 16 h more → Sunday 1600.
Watch Out
Add the flight time to the clock of the city you LEFT. Mixing the two clocks in one sum gives nonsense.
Explanation
Flying west the clock seems to go backwards. Leave Sydney at ten on Monday morning, fly fourteen hours, and it is midnight Monday in Sydney; but Los Angeles is seventeen hours behind, so it is seven on Monday morning there — three hours before you left, by the clock, though fourteen hours really passed. Flying west you gain clock time, flying east you lose it. The same two steps every time: flight time on the departure clock, then convert.
Your Turn
Leave Perth at 0600, fly 5 h to Sydney (2 h ahead). Sydney clock on landing? Check: 1100 Perth = 1300 Sydney.
Watch Out
Landing "before you left" is real by the clock. Do not "fix" it — check the two steps and trust them.
Explanation
The whole lesson in four lines: ahead, add; behind, subtract; a flight goes on the departure clock first and is converted once; and the day name is written at every step. If Sydney is seventeen ahead of Los Angeles and two ahead of Perth, then Perth is fifteen ahead of Los Angeles. A call at seven in the evening from Sydney reaches Los Angeles at two in the morning, which is worth knowing before dialling. Quiz and exercises now.
Your Turn
Sydney is 17 h ahead of LA and 2 h ahead of Perth. How many hours is Perth ahead of LA? Check: 17 − 2 = 15.
Watch Out
Read WHICH city the answer is wanted in. A landing time in LA time and in Sydney time are 17 hours apart.
📌 Key Points — this lesson in a nutshell
- Places to the east are ahead: Sydney is 2 h ahead of Perth, Auckland is 2 h ahead of Sydney. (No daylight saving in this lesson.)
- Ahead → add the difference. Behind → subtract. Sydney 3 pm is Perth 1 pm and Auckland 5 pm.
- A big difference can change the day: Sydney is 17 h ahead of Los Angeles, so Sydney Monday 0800 is LA Sunday 1500.
- A flight: work in the departure city's time (departure + flight time), then convert to the arrival city once at the end.
Step 1: Introduction
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