When did the world’s population reach six billion?

F: Professor Lee, the world’s population passed six billion on Octoberl 1, 1999, we are told. How long did it take to get there and how long will it take to reach seven billion? And ten billion?
M: The world’s population was 300 million at the dawn the Christian era, what we now call the year IAD. In AD1500, the world’s population reached 500 million. It took 304 years until 1804 to reach one billion, another 123 years until 1927, to reach two billion and then only another 33 years until 1960, to add another billion to reach three billion.
F: It’s very impressive that the recurring milestones of another billion added to the world’s population occurred quite quickly.
M: Indeed. The world population reached four billion in 14 years in 1974; five billion in 13 years in 1987, and six billion in 12 years in 1999. Data suggests that a world population of seven, eight, and nine billion will take respectively another 11,12 and 14 years to achieve. The world’s population should hit ten billion mid-way through the 21 century in the year 2054.