GLaDOS' take-over: 1900's
Black Mesa incident: 200-
--- I believe one of the writers stated that GLaDOS's takeover of the Aperture Science facilities took place just prior to the Black Mesa Incident and the ensuing resonance cascade. This was to explain to a fan's question as to why the police/government didn't try to rescue the trapped employees at Aperture Labs and shut down GLaDOS.
Both happened in 2003.
Whatcha talkin about, brotha?? lol