In Australia there are three time zones: Eastern Standard Time in the eastern states, Queensland,
New South Wales, Victoria, Tasmania and the ACT; Central Standard Time, in South Australia
and the Northern Territory; and Western Standard Time, in Western Australia.
Compared to Brisbane on the east coast, where I live, Central Time is half an hour behind
Eastern Time, and Western Time is two hours behind Eastern time.
So there's an hour and a half between Western Time and Central Time.
Actually, it's a little more complicated in practice, because Broken Hill in New South
Wales follows Central Standard Time, and there are even some places in the eastern part of
Western Australia that have their own time zone, between Western Time and Central Time.
But I'm going to ignore those issues here.
The official labelling of these times zones is actually by comparing them to something
called "Greenwich Mean Time", the standard reference time for all time zones around the
world.
Greenwich is a place near London (actually within greater London), and it's used as the
reference for time for historical reasons, which I won't go into here.
Western Standard Time is GMT plus eight, meaning eight hours ahead of Greenwich.
Central Standard Time is nine-and-a-half hours ahead of Greenwich.
And Eastern Standard Time is ten hours ahead.
GMT is also sometimes called UTC, which stands for "Coordinated Universal Time".
Yes, those letters are in the wrong order.
That's because the acronym "UTC" is a compromise between English, French and technical usage.
Now in Australia, some states have Daylight Saving.
During the summer months, the official time is moved one hour further ahead.
Eastern Daylight Time is GMT plus eleven, one hour ahead of Eastern Standard Time at
GMT plus ten.
Central Daylight Time is GMT plus ten and a half, one hour ahead of Central Standard
Time of GMT plus nine and a half.
However Western Australia, the Northern Territory and Queensland do not have Daylight Saving
time.
The exact dates when Daylight Saving begins and ends vary a little from year to year,
and sometimes from state to state.
Let's try solving some problems involving
time zones.
Hannah flies from Sydney to Perth, leaving at 2:40pm on a Friday in June.
The flight takes four hours and thirty minutes.
When will she arrive in Perth?
The trick to solving this kind of problem is to work everything out in one time zone.
It doesn't matter whether you use the origin or the destination, but to avoid confusion,
always choose one and stick to it.
I'll use the destination time zone for all the problems in this video.
First question, is there daylight saving?
No there isn't, because it's June, which is winter here in the southern hemisphere.
So Sydney is two hours ahead of Perth.
Now, Hannah departs at 2:40pm Sydney time.
What time is that in Perth?
Well, Perth is two hours behind Sydney, so at 2:40pm in Sydney, it's still only 12:40pm
in Perth.
Now add the four hours thirty flight time: 12:40 plus four hours is 4:40, plus another
thirty minutes is 5:10pm.
Hannah will arrive in Perth at 5:10pm local time.
Let's try another one.
Karen flew from Darwin to Melbourne, leaving at 11:45pm on a Sunday in January, arriving
on Monday at 5:40am.
How long was the flight?
First, is there daylight saving?
Yes there is, because January is the middle of summer in the southern hemisphere.
However, we only have daylight saving in Melbourne, not in Darwin, because there's no daylight
saving in the Northern Territory.
So for this question, Melbourne is one and a half hours ahead of Darwin.
Karen departs at 11:45pm on Sunday, Darwin time, which in Melbourne is one and a half
hours later, or 1:15am on Monday.
She arrives at 5:40am on Monday.
1:15 to 5:15 is four hours, plus another twenty-five minutes gets us to 5:40am.
Karen's flight time was four hours and twenty-five minutes.
In the bigger picture, of course, the whole
globe is divided up into different time zones.
As I mentioned before, they're all measured relative to Greenwich near London.
Australia is between eight and ten hours ahead.
The Americas are many hours behind Greenwich.
In between there's a special imaginary line called the International Date Line.
Strange things happen if you cross the Date Line.
Think about it.
On one side of the Line, it's twelve hours ahead of Greenwich, but on the other side
it's twelve hours behind.
Remember, the world is really a sphere, not just a flat map.
We draw it flat, but it's really a sphere.
Suppose it's Sunday afternoon in Australia.
Then at the same moment, ten hours back, it's Sunday morning in Greenwich.
But in the US, eight hours behind that, it's actually still Saturday evening!
Let's try a simple problem first that doesn't involve crossing the Date Line.
Amy leaves Brisbane at 5pm on Friday.
She lands in London twenty-five hours later, after a stopover somewhere in Asia or the
Middle East.
What day and time is it in London when she arrives?
Well, Brisbane is at GMT plus ten, and London is at GMT.
So Brisbane is ten hours ahead of London.
Let's work in the destination time zone again.
Amy leaves at 5pm on Friday, Brisbane time.
Ten hours before that is 7am on Friday, London time.
Amy's flight lasts twenty-five hours.
Add twenty-five hours to 7am and it'll be 8am on Saturday in London when she arrives.
Last problem.
This one involves the International Date Line.
Nicholas flies from Brisbane to Los Angeles, leaving at 11:30am on Tuesday.
It's a twelve hour flight with no stops.
When will he arrive in Los Angeles?
Again, Brisbane is GMT plus ten, and if we have another look at our time zone map, Los
Angeles is at GMT minus eight.
Brisbane is ten hours ahead of Greenwich, and Los Angeles is eight hours behind that.
Which means there's an eighteen hour time difference.
Brisbane is eighteen hours ahead of LA.
So, when Nicholas departs Brisbane at 11:30am on Tuesday, it's eighteen hours earlier than
that in LA.
Twelve hours back would be 11:30pm on Monday, and another six would make it 5:30pm on Monday
in LA when Nicholas leaves Brisbane.
Add the twelve hour flight time, and it will be 5:30am on Tuesday when he arrives in Los
Angeles.
Look at that: he arrives before he departs, in local time!
It seems strange, but it's only because he crossed the International Date Line from west
to east.
He didn't really arrive before he left; it's just earlier in the local time zone at his
destination.
What do you think will happen on his return flight back across the Date Line, from east
to west?