1987: The United Nations designated 11 July as the day on which the world’s population would exceed 5 billion. It was impossible, of course, to pinpoint the date – the Day of 5 Billion was largely symbolic. More credibly, though less precisely, population experts estimated that the number of human beings reached 2 billion sometime during 1927, 3 billion during 1960 and 4 billion in 1974.
Source: Geoffrey Gilbert, World Population: A Reference Handbook (2001), pp. 35–43