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A boy celebrated his birthday with some friends. Strangely, 2 days later, his elder twin celebrated his birthday with the same friends. Now those friends got confused and asked why the order was different, and the twins showed the correct times of their births on their birth certificates. So, the friends were satisfied.

Explain the situation.

Clue:

I made this puzzle by combining two different puzzles. So, if you have solved many such puzzles, then try to combine those answers.

Even though I made this puzzle independently, I now see that somebody else has already made it around 12 years ago.

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  • $\begingroup$ possible duplicate of Older twin's birthday is one day after younger's - this puzzle is 2 days rather than 1 day, otherwise they seem the same. It's a matter of opinion whether that's enough difference to make it a different puzzle, clearly. $\endgroup$
    – A E
    May 23, 2015 at 20:52
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    $\begingroup$ @AE The older question needs the 'aha' of the International Date Line, while this one needs both that 'aha' and the one of February 29th. I've voted to leave open. $\endgroup$
    – Rand al'Thor
    May 23, 2015 at 21:32
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    $\begingroup$ It now seems that there is one "related" [but not Duplicate] question : puzzling.stackexchange.com/questions/8813/… ; but somebody who has solved that will not automatically solve my puzzle. In my clue, I refer to two puzzles (which were combined to make my Puzzle) and the existing puzzle may be taken as one of them. $\endgroup$
    – Prem
    May 24, 2015 at 10:09

3 Answers 3

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The twins were travelling eastwards across the International Date Line during their birth.

The elder twin brother was born first, on March 1st.

After they crossed the International Date Line, the younger brother was born, on February 28th.

When they celebrate their birthdays on a leap year, they're 2 days apart!

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    $\begingroup$ @Doorknob, re your edit - surely British English is acceptable to use on Stack Exchange? $\endgroup$
    – Rand al'Thor
    May 23, 2015 at 19:38
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    $\begingroup$ @randal'thor Sorry, didn't know that was a proper spelling; the two-l's version looked wrong to my eyes and to my spell-checker. $\endgroup$
    – Doorknob
    May 23, 2015 at 20:28
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There's room even for one more day of difference, that is, the elder brother can celebrate his birthday 3 days after the younger brother does.

leoll2 says in his answer that they travel across the International Date Line to jump back to the previous day in the calendar. You don't have to travel to that part of the globe, you can do this at any time zone boundary around midnight (and in fact I have already done this :)).

According to various sources, including http://www.timeanddate.com/time/map/, http://en.wikipedia.org/wiki/Kiritimati, or the tzdata package shipped by many Linux distributions, there are time zones from UTC-12 through UTC+14. (Note: the tzdata package uses opposite signs.) This means that in a non-leap year you could travel back e.g. from Mar 1 00:xx @UTC+14 (Christmas Island) to Feb 27 23:yy @UTC-11 (the sea nearby) or even to Feb 27 22:zz @UTC-12 if you have a really fast plane.

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  • $\begingroup$ +1 , @leoll2 gave the expected solution , but your solution adds a new twist, where there is one more point : any day of the year, we can make a 2-day difference between twins. On leap years, we get the opportunity for 3-day difference, so maybe you can add this as a new question. $\endgroup$
    – Prem
    May 1, 2015 at 3:27
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    $\begingroup$ I guess I should've posted this as a question at the first place, mostly aimed for you two guys :) Next time I'll do so. $\endgroup$
    – egmont
    May 1, 2015 at 22:25
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    $\begingroup$ @Prem My first thoughts where about time zones and before and after midnight to achieve a two day differences and did not think of leap days. It would be funny though if parents would do this when they are having twins. $\endgroup$
    – fibonatic
    May 17, 2015 at 3:24
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Part 1:

I saw a puzzle where birthdays where in order, but with a gap of 1 day, because of leap year.

Part 2:

I came across another puzzle where the order of birthdays was reversed, because of crossing the "International Date Line".

Combining the two parts:

The first kid was born on 1st March, and the second kid was born on 28th February because of crossing the "International Date Line", so the order will always be reversed, but on leap years, the gap will be two days, with February 29th coming between the two events!

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