When did the new Millennium start?
The first day of the new Millennium was 1 January 2001. The year 2000 was actually the final year of the previous Millennium. So, despite the fact that it was the year 2000, it was officially the 20th century for one more year.
Where did the new Millennium start and why?
The new Millennium officially started at the Prime Meridian measured from the Royal Observatory, Greenwich.
At the 1884 International Meridian Conference, a resolution was agreed which established the formulation of a 'universal day' calculated in terms of mean solar time, i.e. the average of a year's worth of days. This is a scientific time scale used irrespective of time zones. The universal day is measured from the Prime Meridian in Greenwich, which is therefore the point from whence each new millennium will begin.