Simplify the following text for a Grade 6 native English-speaking person.
Original: Helen Elizabeth Clark , ONZ ( born 26 February 1950 ) is the Administrator of the United Nations Development Programme , the third-highest UN position , and was the 37th Prime Minister of New Zealand for three consecutive terms from 1999 to 2008 .
Simplified: Helen Elizabeth Clark ( born February 26 , 1950 ) was the Prime Minister of New Zealand . She became the PM in December 1999 .