at 01:46:40 utc on 9 september 2001, the unix billennium (unix time number 1,000,000,000) was celebrated
a billennium is an event celebrated every billionth second from the time since 00:00:00, 1 january 1971
"at 01:46:40 utc on 9 september 2001, the unix billennium (unix time number 1,000,000,000) was celebrated" What is the definition of "billennium"?