2nd millennium
The 2nd millennium began on January 1, 1001 and ended on December 31, 2000. The Julian calendar was used in Europe at the beginning of the second millennium, and many countries that continued using the Julian calendar had adopted the Gregorian calendar by the end of the second millennium before it ended. The end date is only calculated according to the Gregorian calendar, while the beginning date is calculated according to the Julian calendar, and sometimes the proleptic Gregorian calendar.