Hi Massimo,
There is some disagreement about the year zero convention... details
on how Guide handles it are in the users manual:
http://www.projectpluto.com/manual/page42.htm
More information is at:
http://www.projectpluto.com/calendar.htm#y2000
Side note: In the course of selling Guide, I've had a few... um...
interesting people contact me: the gent who had proven the Bible was a
message sent from the Orion Nebula; "Arc Angel", a person in Australia
who sent me a densely-written scrawled page telling me God would punish me
and all like me for writing astronomy software; two or three astrologers
with peculiar theories of one sort or another; one person who wanted to
prove that an imaginary asteroid was about to collide with the earth. But
such people are rare.
The above "y2000" link, though, brought forth four or five e-mails
from people who were _really_ annoyed at the idea that there might be
a year zero. There is something about it that brings forth weird
behavior. Somebody looking for a PhD thesis in psychology should look
into this.
-- Bill