Simplify the following text for a Grade 9 native English-speaking person.
Original: He was principal conductor of the London Philharmonic Orchestra from 1967 to 1979 , music director at Glyndebourne Opera from 1978 to 1988 and at the Royal Opera House , Covent Garden from 1987 to 2002 .
Simplified: Outside of the Netherlands , Haitink was principal conductor of the London Philharmonic Orchestra from 1967 to 1979 . Haitink also served as the music director at Glyndebourne Opera from 1978 to 1988 .