The 19th century lasted from 1801 to 1900 in the Gregorian calendar (using the Common Era system of year numbering). Common usage sometimes regards it as lasting from 1800 to 1899, but this is considered incorrect due to the nonexistence of a "Year Zero" before AD 1. The 19th century is also sometimes known as the eighteen hundreds (1800s), referring to the latter usage. Decades are almost always considered as starting with the "0" year and named accordingly ("1890s", etc.), so the first decade of a century technically overlaps back into the preceding one.