How did Bukhari verify the birth/death dates of narrators

One aspect of authenticating a hadith is to make sure that there is a continuous chain of narrators. So this means looking at biographies of the narrators to find out when they were born and when they died. e.g was narrator A alive at the same time as narrator B. My question is though, how were these dates verified either by the authors of those biographies or Bukhari himself?