When is my birthday?

June 20, 2007 4:09 AM

They tell us that future travelers in high-speed spaceships will have their perception of the passage of time distorted by relativity effects, but what about the more mundane problem of earth-bound time zones? I can't figure out when my birthday is...

I was born in England, at (so they tell me) around six in the morning. When I was five, our family emigrated to Canada, a number of timezones westward. No big problem. But I now live in Japan - and crossed the International Date Line to get here. So I should perhaps add one day to my birthday, and celebrate it one day 'later', to try and keep it lined up with reality.

But here's the problem - a few years ago I made a round-the-world trip (multiple stops) with my family, flying west all the time, and thus crossed the Dateline yet again in the same direction as we returned to Japan. So am I now two days ahead? When is my birthday?

posted by woodblock100 to travel & transportation (16 comments total)

My dictionary defines Birthday as "the annual anniversary of the day on which a person was born".

So you're celebrating the day you were born, not the quantity of time that's passed for you personally since you were born.

I assume that ships travelling so fast that time becomes distorted will keep their own calendars, probably based on Earth's because of circadian rhythms.
posted by humblepigeon at 4:25 AM on June 20, 2007

So am I now two days ahead? When is my birthday?

If birthday presents are involved, the answer is that you now have three birthdays per year. :)

I share a birthday with a family member. Travel and the dateline means that we no-longer share our birthday. Rather than worry about when is correct, I just use the latitude to have my birthday on the day I most want it to be. It helps if you want a party on the weekend.

A more direct answer to your question is that crossing the dateline does not affect the time that has passed - if you were to travel around the world once per day, every day, the calendar would not stop advancing, likewise in the opposite direction, the calendar would not advance twice as fast such that a year of dates passes in six months for you.

As such, you are not two days ahead. Just celebrate it when the date of your birthday occurs in England if you want to be accurate.
posted by -harlequin- at 4:35 AM on June 20, 2007

Yeah, it's not time travel, it's timestamp travel.
posted by peacay at 4:51 AM on June 20, 2007 [2 favorites]

If you go past the dateline twice in the same direction you must have passed a midnight at least once, and this balances out one of the days, right?

Otherwise, if you want to know when your birthday is you can just use the time and date you were born in UTC to work out when however-many-years have passed in whatever time zone you happen to be in.
posted by edd at 5:05 AM on June 20, 2007

Just convert your birth time to Unix Epoch-Seconds. What could be easier?
posted by odinsdream at 5:14 AM on June 20, 2007

You're not actually traveling in time. Don't worry, your birthday stays at the same time.

How about those people born on February 31st? Wow... I sure pity them.
posted by remi at 5:14 AM on June 20, 2007 [1 favorite]

Your birthday is the same date it's always been using whatever the local calendar is. remi, unless I don't get the joke I think you mean Feb 29th.
posted by monkeymadness at 5:28 AM on June 20, 2007

I reckon you're right, and you should celebrate when that time happens at your birthplace, whatever the date may be for you locally.

I always wanted to go to a pole and run around and around....
posted by pompomtom at 5:52 AM on June 20, 2007

The RTW trip is meaningless for this quesiton. The whole point of the date line is to remove the accumulated fractions of a day that you gained in each time zone. A RTW trip results in a net change of zero.
posted by smackfu at 6:28 AM on June 20, 2007

Your history of travel matters not one bit.

All that matters is where you were born and where you are now.

You can feel free to celebrate your birthday in your local time or based on the time it is at the place you were born. Or both. Thats a completely personal preference.
posted by vacapinta at 6:32 AM on June 20, 2007

This question does point out how few people actually understand the Date Line and its purpose.

If it weren't for the Date Line then I could just get on an airplane and fly so as to keep the Sun in the same position in the sky. If it is 10am now it will always be 10am. And so Time would never move forward!

Except that at some point I cross the Dateline and 10am June 20 suddenly becomes 10am June 21.

It exactly solves this type of problem.
posted by vacapinta at 6:39 AM on June 20, 2007

Hey, Dave Bull (woodblock100) the woodblock print maker in on Metafilter! Yay! I came across your site via purelandmountain several years ago. Happy birthday, somewhere in time!
posted by KokuRyu at 7:42 AM on June 20, 2007

If you were born on a particular date in England, why not celebrate your birthday on that date wherever you are? Even if that does mean you're in Japan and it's actually a day earlier in England?

But if you want to calculate when you are exactly N years old, that would be based on whatever time it is in England.

As a side note, my wife was born June 8 in Japan, and celebrates her birthday on June 8 though we are now in the US. Her grandparents in Illinois were told as soon as she was born; it was earlier here in the States, and they still think her birthday is June 7.
posted by rossmik at 9:08 AM on June 20, 2007

PS Date line problems are easier to think of if you never think of "crossing the date line." Instead of "I went 8 time zones west, but crossed the date line, so I go back 8 hours and forward a day", just think "I went 16 time zones east (even though I took a westward shortcut to get there)."

On my mental timezone map, the date line is at the edges.
posted by rossmik at 9:17 AM on June 20, 2007

Yeah, your birthday is the same date it was when you were born. My stepdaughter was born in Japan. It was almost certainly the day before here in Alaska, but we celebrate on the day that shares the date on her birth certificate, not the day it was where we are exactly x years before.

You can't gain or lose days that way (crossing the date line). You've lived the same number of days as everyone else since the day you were born. If you really want to alter it by time zone, though, go by the date your mom's in if she's alive (mine calls me at the anniversary of the minute I was born, which has never yet resulted in us being in a different date just then, but if I got that call in a time zone where it was the day after my birthday date, I'd probably consider that day my birthday).
posted by Cricket at 2:20 PM on June 20, 2007

Just convert your birth time to Unix Epoch-Seconds. What could be easier?

This is the one that nails it most concisely of course - use a definition of time that ignores time zones.

The question wasn't meant to be so deeply serious; I do understand that I have lived the 'same number of days' since I was born as anybody else born at the same time, no matter which way around the world we may have travelled. I was trying to find out how people - now that we all do so much international living and travelling, and have family members scattered here and there - deal with the question of 'which day to celebrate the birthdays on?'

As a perfect example - my grandson was born in Canada, is currently living in Romania, and here I am in Japan ... so complications do arise! And because I live near the 'front edge' of the progress of 'time' around the planet, I'm the one who is supposed to remember these events, and start the phone call merry-go-round!

And the International Dateline does 'bite' occasionally. We flew from Japan to New Zealand in February, but because our Japan-based travel agent had assumed it was across the date line (which it isn't), found when we arrived in Auckland that the onward connecting flight on which we were booked had left 24 hours earlier! Next time, we'll inspect our tickets a bit more closely ...

Thanks for all the replies!
posted by woodblock100 at 3:30 PM on June 20, 2007