If what you are saying is correct there is 1999 years between 1000 BCE and 1000 AD that means that one or the other is the true Millennium. I am not sure how you can celebrate an event 1000 years befor it starts.

Actually the Catholic Church which started the calendar in the first place celebrated the first Millennium when the calendar changed from XMXCIX to X.

Actually since the gregorian calendar didn't start untill the year 532 AD the true Second millennium is still 532 years away.

Paul