Simplify the following text for a Grade 7 native English-speaking person.
Original: In contemporary history , the third millennium is a period of time that commenced on January 1 , 2001 , and will end on December 31 , 3000 , of the Gregorian calendar .
Simplified: The 3rd millennium started on January 1 , 2001 , and will end on December 31 , 3000 .