Answer 1:
When the Gregorian calendar was established, the
system of leap years was included and since then,
we have been messing around with the system to see
that it is exact. For instance, according to the
U.S. Naval Observatory (the official time keeper
for the United States. There is a good website on
all of this at
http://psyche.usno.navy.mil/millennium/ ). A leap
second was added between 1998 and 1999 to even out
the time. The reason for all of this time
juggling is that to keep our arbitrary system of
time accurate, continual allowances must be made
for the fact that the Earth travels around the sun
(making a year) but is also rotating (making a
day) and these rotations don't correspond exactly
(i.e. there are not exactly 365 rotations of the
Earth (days) in one rotation around the sun (a
year)). I would suggest that you look at the
website given above as it gives a very good
explanation of leap years and the historic reasons
for having them. The basic answer to the
question is that every fourth year is a leap year
except centuries that are not divisible by 400 so
2000 is a leap year while 1900 and 2100 are not.
The leap year slightly overcompensates for the
actual difference between the number of days in a
year and the time it takes to go around the sun so
there is never a need to add a second leap day.

Answer 2:
This is an answer to the February 2000 question; I
cannot support the data with references since I
remember this from having read it a while ago and
I don't remember the source. Also, the information
that I give may be inaccurate (I don't remember
how reliable the source was), but it may be good
as a second opinion to contrast to whatever other
people contribute:
The rotation of Earth
around the Sun takes, in fact, approximately
365.25 days, resulting in one extra day each year.
However, this is not exact; there are calculations
available with more significant figures that allow
us to say that it's actually slightly higher than
365.242. This means having around 146097 days
every 400 years, which amounts to 97 extra days
every 400 years, or one extra day every four years
except in three occassions, which are the
centenials not divisible by 400. This can be
summarized as: Rule:365 days per
year. Exception #1:366 days per year divisible
by 4. Exception #2:365 days per centennial
year. Exception #3:366 days per centennial year
divisible by 400. Again, this doesn't amount to
full exactitude in the count of days per year, but
it's a very good approximation. I found all the
information above in a spanish magazine of general
knowledge about 7 years ago, and I found it so
interesting that I memorized it. Any inexactitude
can be blamed either on the publisher of the
magazine or on my own memory, as you choose.
Thinking about it now, I have the following
comments:
+Instead of adding 2 days to the
year 2000, wouldn't it be easier to include
millenia in Exception #3? This would result in the
same numberof extra days per 2000 years, and make
much more sense to me; talking about that: anybody
knows whether year 1000 had 28 or 29 days in
February?
+The reason of all this hussle:
the rotation of Earth around the Sun does not take
an exact number of days, and we try to compensate
it; if you think that this is complicated, think
what would had happened if instead of ~365.242
days, it took about 365.077 days. Then, one day
would have to be added every 13 years. And on top
of that, still we would need to compensate every
God knows how many years... Aren't we lucky that
a simple number such as 4 is the best close
approximation?
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