The 21st century is the present century of the Anno Domini (common) era, in accordance with the Gregorian calendar. It began on January 1, 2001 and will end December 31, 2100. A common misconception is that it started in 2000 and will end in 2099, but due to the fact that there was no year zero, 2000 is part of the 20th century despite all the millenium countdown clocks and millenium parties which celebrated a year early.
Technologically, the major differences from the 20th century are the changes brought about by the digital revolution of the 1980s and 1990s. The 20th century was marked by the rapid development of travel fueled mostly by petroleum, the 21st century is marked by concern over how to cope with consequences of pollution and resource depletion.