1842 (MDCCCXLII) was a common year starting on Saturday of the Gregorian calendar and a common year starting on Thursday of the Julian calendar, the 1842nd year of the Common Era (CE) and Anno Domini (AD) designations, the 842nd year of the 2nd millennium, the 42nd year of the 19th century, and the 3rd year of the 1840s decade. As of the start of 1842, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.