From the degree of variation between two individuals they can tell how genetically distant they are from each other. This genetic distance translates into a time, namely the time in the past when they had a common ancestor. This mapping from genetic distance to time is not completely unproblematic: It is based on the assumption that genetic mutations take place at a constant rate. It is known, however that evolution is punctuated: There is little mutation going on over a long stretch of time and then there is a sudden burst of variation, followed by another quiet period. For that reason very sophisticated statistical tools have been developed to estimate the most likely time in the past when a separation between genetic variants took place.
Bette Korber of the Los Alamos National Laboratory used their Nirvana supercomputer to pinpoint another out-of-Africa event: The origin of the aids virus that infects more than 40 million people around the world was believed to be located in Africa and started to infect humans around 50 years ago. Korber's extensive computational analysis could assign a high probability that the aids virus moved from chimps to humans at a single occasion (therefore that virus is named "HIV-Eve") around 1930, possibly earlier if it remained in a small group. This information will be useful to develop strategies to contain and eventually subdue this dangerous virus.
Computer analysis dates HIV virus to 1930 , Nando Media, Associated Press HIV Sequence Database