I am about to fly from New Zealand to the United States, and on some flights you land on the same date you left. It sounds like time travel. It is actually time zones and one very strange imaginary line in the Pacific Ocean.
I am about to fly from New Zealand to the United States, and there is something slightly ridiculous about my flight itinerary.
I can spend hours and hours on an aeroplane, cross most of the Pacific Ocean, and then look at the clock when I arrive and discover that according to the calendar, hardly any time has passed.
On some flights, you can even arrive on the same date you left.
This sounds suspiciously like time travel.
Unfortunately, I have checked, and nobody is giving me a Nobel Prize.
The real answer involves time zones and one particularly strange imaginary line in the Pacific Ocean.
## First, there isn't actually one time everywhere
Earth takes roughly 24 hours to rotate once.
That means when the Sun is high in the sky over New Zealand, somewhere on the other side of the planet it is the middle of the night.
If everyone used the same clock, "12 noon" might mean lunchtime in Christchurch but midnight somewhere else.
So the world is divided into time zones.
New Zealand Standard Time is 12 hours ahead of something called Coordinated Universal Time, or UTC. During daylight saving, our clocks move another hour ahead.
Even that isn't as tidy as it sounds.
The Chatham Islands are 45 minutes ahead of mainland New Zealand.
Not one hour.
Forty-five minutes.
Apparently whoever designed world time decided whole numbers were getting boring.
## Then there is the International Date Line
Time zones solve one problem but create another.
Imagine travelling west around the world and moving your clock forward every time you enter a new time zone.
Eventually you would have moved it forward by an entire day.
So somewhere on Earth, the calendar has to change.
That somewhere is the International Date Line.
It runs roughly north to south through the Pacific Ocean, near 180 degrees longitude.
If you cross it travelling west, you move forward one calendar day.
Cross it travelling east, and you move back one day.
Which is what happens when I fly from New Zealand towards the United States.
I haven't actually travelled backwards through time.
My birthday isn't getting further away.
The plane is simply moving from one side of the world's calendar to the other.
## Why isn't the line straight?
This was the bit I found most interesting.
The International Date Line is imaginary.
There isn't a giant stripe painted through the Pacific, although I think that would make flying over it much more exciting.
It also isn't completely straight.
It bends around countries and groups of islands so that people living in the same country don't unnecessarily wake up on different dates.
Kiribati, for example, changed which side of the date line some of its islands use so the whole country could share the same calendar day.
So although Earth's rotation is science, the way humans divide time is partly a decision.
Seconds are measurements.
Time zones are agreements.
## Your phone makes all of this look much easier than it is
Today I can land in another country, turn on my phone and within seconds it usually knows the local time.
That hides an enormous system underneath.
Countries have rules about their time zones.
Some use daylight saving.
Some don't.
Some use offsets of 30 or 45 minutes instead of whole hours.
And those rules can change.
Your phone is basically doing international clock homework in the background so you don't have to.
## So am I really arriving before I left?
No.
Sadly.
If I spend 13 hours on an aeroplane, I have still aged 13 hours.
The clock at my destination is simply measuring those hours from a different starting point.
But I still think there is something pretty amazing about leaving one country, crossing an invisible line in the ocean and watching the calendar move backwards.
Humans didn't invent time.
But we definitely invented some wonderfully complicated ways of keeping track of it.