The 21st century officially started on January 1, 2001. This is because the Gregorian calendar, which counts years starting from 1 CE (there was no year 0), means each century begins with the year ending in 1 and ends with the year ending in 00. Hence, the 20th century was from 1901 to 2000, and the 21st century began on January 1, 2001, and will end on December 31, 2100.