birthdays in different time zones

howdy peeps

can you help with this question?

I have an acquaintance who lives in Sydney. We keep in contact via Facebook. conveniently, facebook reminds me when it is his birthday (October 12th). we’re not close friends or anything, you understand, but it is still nice to send a quick greeting.

he was born in the UK. so my question is about when his birthday actually is

a search online has resulted in mostly obvious and easily acceptable answers such as ‘he can celebrate his birthday on whenever October 12th occurs in that time zone’ etc. etc. and are generally in relation to bureaucratic situations (date of birth for driving licences or passports, for example)

but i’m looking at the bigger picture. My thinking is that the moment of his birth occurred in a specific ‘space’ and at a specific ‘time’ which are relative to each other.

therefore, if he was born in the UK, then his birthday (those special 24 hours of 12th october) will never change. it will always be the same in the UK, however, if he moves time zones (10 hours ahead to Sydney), then his birthday (relative to him in Sydney) actually begins at 10am oct 12th and ends on 10am oct 13th?

am I correct?

I wouldn’t be too certain about that. Suppose your friend was born on October 12, 1996. He would turn 18 and thus become of legal age on October 12, 2014. Now suppose he concludes a contract (for which the age of majority is required) in Australia on October 12, 2014, 1am local time, at a time when it is still October 11 in the UK. I don’t see a reason why Australian law should not consider that contract to be legally valid and enforceable, since your friend was, as far as Australia is concerned, 18 at the time. Similarly, if your friend commits a crime in Australia on October 12, 2014, 1am, I don’t see a reaqson why Australia could not try him under adult, as opposed to juvenile, law (assuming that adult criminal law becomes applicable at the age of 18).

Which probably only goes to tell you that the answer may vary depending on the purpose for which you want to determine the exact birthday.

His birthday is wherever he is on that date.

Think of New Year’s Eve/New Year’s Day. Everything is local. The Midnight kiss-the-person-you-are-with-at-Midnight occurs hourly across the globe, at each location, until the entire planet has entered the new year.

Or look at it another way. Your friend’s birthday (October 12th) is the day he boards a jet in Sydney (SYD) bound for Los Angeles (LAX). So he has a few great hours partying with friends on his birthday before he leaves. Upon arriving at LAX he’s greeted by other friends, on his birthday.

Other friends. Other location. Same day. Everything is relative to location.

You’re over-thinking all this. A “birthday” is a pretty squishy concept. Specifically …

These two paragraphs & ideas are mutually inconsistent.

Let’s pretend your friend was born at 9am on Oct 12th. And let’s pretend he never leaves the time zone of his birth for the rest of his life.

Therefore 8am on Oct 12th of the year he was born is not his birthday. How could it be? He wasn’t born yet.

When some later year rolls around to Oct 12th, how can it be the guy’s birthday at 8am? It isn’t the anniversary of his birth yet.

So if you want to define a “special 24 hours”, they need to be the 24 hours from 9am on the 12th to 9am on the 13th.

And so if your friend is now in a different time zone, and you want to remain faithful to the true anniversary, you need to convert 9am on the 12th & 9am on the 13th to whatever they are in local time and celebrate that time interval.

Now if all that sounds silly & too much trouble & quibbling over details, … well, I agree with you.

As does your thought that you should ignore the “9am” part but pay attention to the “different time zone” part of the situation.

IOW, if we’re going to simplify “the special 24 hours” by celebrating the nearest whole calendar day for somebody who stays in one time zone their entire life, we ought to be consistent in that and celebrate the nearest whole calendar day in the nearest relevant time zone for somebody who’s changed time zone.

And the birthday person’s time zone is the determinative one. Where you or I are is immaterial. So time your greetings so he/she gets them on the 12th in his/her timezone.

If you want to consider it in that much detail, then it probably won’t always be on the same date even if he stays his whole life in the house he was born. Because a year is (approximately) a quarter-day longer than 365 days, each year the anniversary of his birth will be 6 hours later in the day. At least, until leap year comes around, when it will instead jump backwards 18 hours to compensate.

Every spring, when my birthday rolls around, I make it a point to calculate just what date it falls on, and at what time.

I give myself as an example. For decades, I have lived in Thailand, 13 or 14 hours (depending on the time of year since Thailand never changes its clocks) ahead of the spot in the Rocky Mountains where I was born. The day I was born in the US, it was actually the following day over here. I still celebrate my birthday on the calendar date that it was in the US instead of waiting for the “proper” date and celebrating it the following day here.

For example, let’s assume I was born on January 1. (I was not.) Due to the time of day I was born in the Rockies, it would have been January 2 here. But celebrating my birthday on January 2 anywhere in the world would feel very odd indeed. When it’s January 1 here, that’s my birthday even though it’s still December 31 back in the US. I don’t think I’ve met a single other transplant over here who does it any differently.

It’s very simple. I was born 29 days before Christmas. So I celebrate my birthday 29 days before I celebrate Christmas. I expect to celebrate Christmas on the same day as everyone else does at the place I happen to be at the time. I’ve spent Christmas in Nairobi, Amman and Singapore, and it is pretty easy to tell what day that is.

In Leap Year, do you celebrate your birthday one day early, or do you wait until the 366th day after the last one, when your age has advanced a year and a day?

I was born in Sydney at about 2 am. At that time, it would have been 4 pm the previous day in England, and 11 am the previous day in Ohio (not allowing for daylight saving). However, when I lived in England (for 7 years) and in Ohio (for 12 years), I treated my birthday as if I was still in the same time zone, so the same day as by the calendar, and not the previous day.

thanks for the answers guys.

ok. lets forget celebrating birthdays (thanks for clearing that up)

i appeciate that time is relative to location.

we are all only born once (lets keep reincarnation out of this for the minute).
then there is only ever one single moment of our birth
so the anniversary of that moment can only ever be relative to the particular place (ergo time-zone) in which that singular event occured.

right? (or wrong? let me know)

You can’t say: let’s forget celebrating birthdays and go on to ask about “anniversary of the moment of our birth”. And you’re putting accuracy in where it doesn’t exist, but for fun I’ll run with it a bit.

Let’s first ignore the movement of the Sun in the Galaxy and the changes in the tilt of the Earth’s axis and the Earth’s orbit and pretend there’s an absolutely accurate description of the Earth’s point in the orbit around the Sun, instead of a relative one. Let’s further, for reasons of simplicity, say that you’re a New Years Baby born in the year 2000 right after midnight. That’s your single moment of birth.

The first anniversary of that moment, if you’re not happy about the inaccuracies of calendars, and using the sidereal year given on Wikipedia, 6:09:09.76 January first 2001.
The second is 12:18:19.52 January first 2002.
The third is 18:27:29.38 January first 2003.
The fourth is 00:36:39.14 January second 2004
The fifth is 06:45:48.90 January first 2005

Is this at all meaningful? No, it isn’t. Anniversaries are a cultural concept connected to whichever calendar you use, they’re not celebrations of some “magical” single moment.

The relevant “place” is of course the point the earth was in the 365-ish day orbit around the sun when you were born. When it gets to that place again, that’s the anniversary of your birth. Depending on local vagaries of measuring time and date, the way that moment in time is expressed in clock and calendar terms will bounce around a bit from year to year, and in the same year will be differently expressed at different spots on the surface of the earth. But even if you never leave the house you were born in, the anniversary of your birth measured in this way will not fall on the precise hour, minute or second of the day per that your actual birth fell on, never mind on the precise calendar day. The hour and the day both depend on the rotation of the earth around its own axis; the anniversary of any event on the rotation of the earth around the sun. These are different cycles which do not align neatly.

If you happen to be in American Samoa on your birthday, you could nip over to Samoa at midnight and have another one. It wouldn’t make you a year older though.

Nitpick: minors may enter into contracts in common law jurisdictions. The distinction is that the contract is voidable at the minor’s election until he reaches majority. That is as distinguished from situations where contracts are void ab initio - that is, they never come into being regardless of anyone’s election - such as when they are against public policy (e.g., a contract to sell yourself into slavery.)

right I see :slight_smile: how intriguing.

i’ll never look at a calendar in the same way. linear time does not seem quite so reliable as I thought.

thanks all!

If you are going to begin with the exact moment of your birth, then your first need to define a year.

Calendar Year?
Sidereal Year?
Tropical Year?
Lunar Year?
Anomalistic Year?

I’m a Kiwi living in the UK. One of the perks is that I start to get happy birthday wishes from my NZ-based Facebook friends the day before my birthday. Makes my birthday last two days!

It’s trickier going the other way though, because I have to remember their birthdays/anniversaries because by the time Facebook tells me, I’m a day late.


Don’t forget solunar year.

I think there’s a simpler solution: convert the birthday, including time to UTC. Your yearly birthday is that UTC date thereafter, wherever you may be.

Solution to what? Adjusting for time-zone just doesn’t make sense.

In the EU, all trucks are fitted with an electronic device that records, among other things, the driver’s name, work time, driving time, breaks, etc. The device shows the time on a panel, which is normally set to local time, but the record is always in UTC. This takes care of any possible anomalies when driving across time zones.

Drivers in the UK will mostly reset their clocks today or tomorrow to take account of the end of summertime. It makes no difference to the record though if they don’t.