Kerry celebrated her birthday.

So the mother was on a boat that sailed over the International Date
Line from March 1 to February 28.  Old hat to people on this news group.

Of course, since then we've had a real-life news story of twins
(single mother, single uterus, single pregnancy) whose birthdates are
several months apart.  Now *there's* a lateral-thinking puzzle.

-- David A. Karr <karr@cs.cornell.edu>

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This post deals with the 'twin birthday paradox' puzzle, whose point is
that the OLDER of two "ordinary" twins has his/her birthday AFTER the
younger of the pair, without biological tricks, fudging the definitions,
etc.  The trick is to get the older twin's birthday as MUCH later as
possible.  Dean Huffman posted the 2-day version, which, sad to say, has
NOT been state of the art :) since I invented the 3-day version last
year.

The two one-day solutions

In order to make this work, the mother has to travel around between the
births of the two twins.  There are two solutions:

A. The date line trick.   The older child is born WEST of the
international date line (any time of day will do); shortly thereafter,
the mother crosses the line going eastward, moves back to the previous
day, and gives birth to the younger child.

B. The time zone trick.  The older child is born EAST of some time zone
boundary a few minutes after midnight.  The mother then crosses the line
going WESTWARD, moves back to the previous day, and gives birth to the
younger child.

The two two-day solutions:

Either A or B can be expanded a day by using

C.  The leap year trick.  The older child is born on March 1 of a
non-leap year; when the mother moves back to the previous day using
either A or B, the younger child is born on February 28.  Then in a leap
year the twins' birthdays are TWO days apart.

The three-day solution:

This is obtained by combining ALL THREE of the above tricks.  The older
child is born on March 1 or 2 in a non-leap year.  The mother then moves
around and loses TWO days by using BOTH the date line trick AND the time
zone trick, and the younger child is born on Feb. 27 or 28, two days
earlier by the calendar and three days earlier in a leap year.

Now WAIT, you may say!  How can the mother go EAST over the
International Date Line and at the same time go WEST over a time zone
boundary?  Well, fortunately for our problem, the International Date
Line is not straight, neither are the time zone boundaries in the
Pacific, and the two sets of zigs and zags do not entirely line up.
There are a few "late" locations on the WEST side of the date line which
are LATER on the clock than some of the "early" locations on the EAST
side of the date line.  Pairs of these locations can be up to 25 hours
apart!!! Yes, for real!

According to my time zone map, late west-side locations (group 1)
include Tonga, the Chatham Islands, and the area around Anadyr, Siberia.
Early east-side locations (group 2) include most of the international
waters just east of the line from south of the Aleutians to (and maybe
including) Howland Island, and also the waters east of the line and
south of the diagonal jog that it makes near the Chatham Islands.  If
twin 1 is born in a group 1 location just after midnight, and the mother
then takes a fast boat or plane so as to get to a group 2 location
before the hour strikes, twin 2 can be born at 11:something p.m. TWO
days before his/her elder sib according to the calendar, which can be
extended to three using the leap year trick.

There it is :)

-- Pete Kimball <achepak@interaccess.com>