The 20th century lasted from 1901 to 2000 in the Gregorian calendar. Common usage sometimes regards it as lasting from 1900 to 1999, but this is considered incorrect due to the nonexistence of a "Year Zero" before AD 1. The 20th century is also sometimes known as the nineteen hundreds (1900s), referring to the latter usage. Decades are almost always considered as starting with the "0" year and named accordingly ("1960s", etc.), so the first decade of a century technically overlaps back into the preceding one.