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I was born 1 day after my sibling. Yesterday was my sibling's birthday but today is not my birthday. When is my birthday?

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    $\begingroup$ Crikey! That was quick! ;) $\endgroup$
    – AJFaraday
    Mar 22, 2018 at 16:40
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    $\begingroup$ you are Siamese twins? $\endgroup$
    – JMP
    Mar 25, 2018 at 9:15

14 Answers 14

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It is

February 29 (and the riddle is being posed on March 1 in a non leap year).

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    $\begingroup$ Instantly my first thought when I read the title. $\endgroup$
    – Tikeb
    Mar 22, 2018 at 15:34
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    $\begingroup$ Or the other way round - your birthday is on 1 March in a non leap year and the riddle is asked on 29 Feb. $\endgroup$
    – IanF1
    Mar 24, 2018 at 21:58
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Yesterday. You were born 24 hours after your sibling, but they were born sometime between midnight and 1 AM of the day that time changes back to standard time from daylight savings time, so your birthday also falls on the same day.

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    $\begingroup$ This is not the intended answer but the reasoning is very interesting $\endgroup$
    – luizfzs
    Mar 21, 2018 at 19:59
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    $\begingroup$ @luizfzs Even if this wasn't the intended answer, it's more ingenious. $\endgroup$
    – Lawrence
    Mar 22, 2018 at 0:14
  • $\begingroup$ @Lawrence I never said otherwise $\endgroup$
    – luizfzs
    Mar 22, 2018 at 11:57
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    $\begingroup$ Except that the riddle says 1 day, not 24 hours. Also, the clocks can change at different times for different countries (1am in Europe, 2am in US, for example). Also, also - then the title would not be true. $\endgroup$
    – sirjonsnow
    Mar 22, 2018 at 12:09
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    $\begingroup$ @sirjonsnow The first couple of weeks of November (when daylight savings ends in the US) are relatively uncommon for birthdays, though admittedly not the most uncommon. thedailyviz.com/2016/09/17/how-common-is-your-birthday-dailyviz. Also, 24 hours is one definition of a day. They can be considered equivalent. $\endgroup$
    – called2voyage
    Mar 22, 2018 at 14:58
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Similar to Daniel's answer but a slight variation:

Your sibling's birthday was 2/28 and your birthday was 3/1, both of you being born on a non-leap year. The current year is a leap year. Your birthday is tomorrow.

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    $\begingroup$ But that ignores the title. :) $\endgroup$
    – Rubio ♦
    Mar 22, 2018 at 9:11
  • $\begingroup$ @Rubio depends on what you consider uncommon. According to this it is about .2% chance for you to have been born this day. $\endgroup$
    – David Starkey
    Mar 22, 2018 at 15:33
  • $\begingroup$ @Rubio, being born on a different day as your sibling/twin is likely to make your birthday uncommon in at least some respect. To be born on the 2/28 - 3/1 time span such that your birthdays are further apart once every 4 years would make it even less common. $\endgroup$
    – Chris Porter
    Mar 22, 2018 at 18:36
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    $\begingroup$ @DavidStarkey Actually in that data set, 11,131 is the average births per day (note, same dataset), and the "your birthday" of this answer, at 11,129 births, is about as average a birth day as you can get. (100% / 365 days is 0.274% chance of any given day being one's birthday. This day's chance specifically is also 0.274%, according to this data.) $\endgroup$
    – Rubio ♦
    Mar 23, 2018 at 7:32
  • $\begingroup$ @ChrisPorter I'm not sure how your birthday's relation to anyone else's birthday makes your birthday more or less common. We're concerned with "my birthday is uncommon", not "the relationship between my birthday and my sibling's birthday is uncommon". :) $\endgroup$
    – Rubio ♦
    Mar 23, 2018 at 7:34
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You were born on one side of the international date line and your sibling was born on the other side of the date line.

so

This means that although you were born 1 day later it may have been either the same day or two days later in terms of the official date - this is a bit like the way if I remember correctly the person going `around the world in 80 days' had an extra day because they went through the date line in a direction to gain a day.

All I can say after that is your poor mother carrying twins then having such a long labour.....

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    $\begingroup$ Nice, thanks for catching the other case I was thinking of. I would have eventually posted this, but since it was so early I wanted others to have a shot at it. $\endgroup$
    – called2voyage
    Mar 21, 2018 at 18:25
  • $\begingroup$ I have added an answer for the case where a country did exactly what you suggest here. $\endgroup$
    – uɐɪ
    Mar 22, 2018 at 14:00
  • $\begingroup$ @ʎəʞouɐɪ yes I had already seen it and upvoted it :-) $\endgroup$
    – tom
    Mar 22, 2018 at 14:23
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Your birthday is

on the 1st of January.

You were born

in Kiribati, on some of the eastern islands. Your sibling was born on 30th December 1994. On the next day, the time zone for the Line Islands has been changed to UTC+14, to unify the date for the country with the western part, thus skipping one day ahead.

Alternately,

you might have been born on a relativistic spaceship, while the labo(u)r took 1 day local time, more days passed according to Earth calendar. But I consider the first version more likely.

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  • $\begingroup$ This is not the intended answer but for sure is an answer. Interesting curiosity about Kiribati! $\endgroup$
    – luizfzs
    Mar 21, 2018 at 20:02
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Today is not your birthday because:

You died last year.

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    $\begingroup$ That's dark. I love it! $\endgroup$
    – Saeïdryl
    Mar 22, 2018 at 16:05
  • $\begingroup$ A date is still your birthday (ie the anniversary of your birth) after you die. But I do appreciate the idea. $\endgroup$
    – pbhj
    Mar 23, 2018 at 23:31
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Another calendar irregularity/discontinuity:

Your sibling was born on:

October 4th 1581

You were born on:

October 5th 1581

Yesterday was your sibling's birthday:

October 4th 1582, the last day of the Julian calendar

Today is not your birthday. It is:

October 15th 1582, the first day of the Gregorian calendar. 10 days were deleted in the transition from the Julian to the Gregorian calendar in 1582

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In addition to the already mentioned possibilities, your birthday may also be on

December 31st or June 30th, just like your sibling's (I feel bad for the mother, that's a painfully long labour..)

Since Earth's rotation isn't constant, the universal coordinated time drifts away from the mean solar time, which is why

occasionally, a leap second is added, making it possible that your sibling was born at 00:00:00 on such a date, and you were born at 23:59:60, which is exactly 86400 seconds (one day) later,

but you still share the same birthday.

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You live in

Samoa.

Your sibling as born on

29 Dec 2011

You were born on

31 Dec 2011

Why this matters

In Dec 2011 Samoa moved its timezone across the international dateline resulting in there not being a 30th December in 2011

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You were both born on a planet with 48 hour days and now live on earth... thus in earth days your birthday is two days past your siblings.

Or how about

You were both born on the east coast, your sibling at 11:55pm, you at 12:05am the next day... You move to sunny California which is three hours earlier. So on west coast time, your sibling was born at 8:55pm and you were born at 9:05am on the same day! So now you and your sibling share the same birthday.

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  • $\begingroup$ Anyone know why I have a -1? $\endgroup$
    – Byte Player
    Mar 22, 2018 at 23:57
  • $\begingroup$ Welcome to Puzzling! People downvote for all sorts of reasons - not enough explanation is a common one, though that doesn't seem to apply to yours - so it's hard to tell why yours got a -1. If you'd like, you can take the tour to learn more about how the site works $\endgroup$
    – puzzledPig
    Mar 23, 2018 at 0:11
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    $\begingroup$ Perhaps because even for lateral-thinking puzzles—which this is NOT—the solver is not given free license to invent their own rules or scenarios. Especially for puzzles not tagged "lateral thinking", the right answer to a puzzle will be the one that uses what the puzzle gave you or hinted at, without inventing facts, rules, or interpretations out of thin air to make a "solution" work. Puzzle posters can't close every loophole… and shouldn't have to. $\endgroup$
    – Rubio ♦
    Mar 23, 2018 at 7:17
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"My sibling" refers to:

A different sibling each time.

For example:

Sibling A was born on, say, 08-Sep-2010.
You were born on 09-Sep-2011.
Your other sibling B was born on 22-Mar-2013 (Birthday Yesterday).

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An alternate answer; very similar to the accepted

I was born 1 day after my sibling.

The sibling was born on February 28, asker was born on March 1

Yesterday was my sibling's birthday

Today is a leap day making it February 29. Yesterday is February 28, the siblings birthday

but today is not my birthday.

It's only February 29, the asker's birthday is still a day away

When is my birthday?

As stated, the birthday in March 1

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    $\begingroup$ You start with the siblings born on 28 Feb and 1 Mar (Chris Porter's answer), but your reasoning seems to rely on them being born on the same day (29 Feb). $\endgroup$
    – Lawrence
    Mar 22, 2018 at 0:19
  • $\begingroup$ @Lawrence, I would like to point out that it's not "Chris Porter's answer" as we posted at the same time. He just managed to hit post while I was formatting. Beyond that, I see why you are confused by the "Reasoning" section. I meant it as "Reason why this is a more logical answer than the accepted" which in re-reading I should have just skipped (plus then it might have been posted pre-Porter) so I'm deleting it. $\endgroup$
    – MivaScott
    Mar 27, 2018 at 21:47
  • $\begingroup$ Yeah, that’s happened to me as well. Here’s a +1 from me. :) $\endgroup$
    – Lawrence
    Mar 27, 2018 at 22:51
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but today is not my birthday.

You were just born

Birthdate vs Birthday

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  • $\begingroup$ It's intentionally written birthday because the specific year doesn't matter. $\endgroup$
    – luizfzs
    Mar 22, 2018 at 20:03
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because

You were born the day after him but you are now a Jehovah's Witness and do not have birthdays.

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