Simplify the following text for a Grade 9 native English-speaking person.
Original: The Kingdom of Great Britain and the rest of the British Empire ( including the eastern part of what is now the United States ) adopted the Gregorian calendar in 1752 ; by which time it was necessary to correct by eleven days ( Wednesday , September 2 1752 being followed by Thursday , September 14 1752 ) .
Simplified: Britain and the British Empire ( including the eastern part of what is now the United States ) adopted the Gregorian calendar in 1752 , by which time it was necessary to correct by 11 days . Wednesday , 2 September 1752 was followed by Thursday , 14 September 1752 .