Re: Was this a lie detecion?


"Marcus Fox" <please-reply-via-newsgroup-th@xxxxxxxxxxxxxxxx> wrote in message
news:lLCdnQx5H9OYhtTbnZ2dnUVZ8tWnnZ2d@xxxxxxxxx
A better question would be "What day of the week were you born on?".
Most people
know this, it doesn't change year to year, and it would much harder
for a
fraudster
to work this out on the fly.

How do you work that out? I have no idea what day of the week I was born
on, and save doing a calendar search, have no idea how to work it out.

1) Find the day of your next birthday, or your last one if it was between
1 March
and now.

2) Add a quarter to your age on that birthday ignoring any fraction (eg
30 becomes
37)

3) Divide by 7 and take the remainder (eg 2 in the example above).

4) Your day of birth was [the number worked out in 3)] days before [the
day worked
out in 1)]

I was wondering if this would work for other dates. Say (hypothetically) I
was born on November 28th 1987, to make me 20 this November coming.

November 28th is a Wednesday, divide 20 by 4 and add, gives 25. Dividing by
7 and taking the remainder leaves 4, so November 28th 1987 should be a
Saturday. It is.

Now say I was to do this before my birthday last year. November 28th 2006 is
a Tuesday. I'm due to be 19, so divide 19 by 4 and add, gives 23. Dividing
by 7 and taking the remainder leaves 2, so November 28th 1987 in this case
is calculated to be a Sunday. But I've already shown above it was a
Saturday.

Odd though, it works for other examples of my actual birthday in different
years.

Yes, it's only for dates between March this year and Feb next year.

The principle is that each year the same date is a day ahead of the previous year,
except for leap years when it's 2 days ahead. So you need to know how many leap
years (more specifically, how many leap days - 29 Feb's) there are between the two
dates. As next year is a leap year, if you go back 3 years from any date between
Mar07-Feb08 you won't have a leap day, if you go back 4 years you'll have 1, if you
go back n years (not to before 1901) you'll have int(n/4) ie n/4 ignoring the
remainder.

From Mar08-Feb09 the same formula will work except you need to round *up* any
fraction from the divide by 4 in stage 2 (eg 30 becomes 38) since going back 1 year
will pass through 1 leap day.

More calendar related geekyness - the 13th of the month is more likely to be a Friday
than any other day.

--
Andy


.

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