1804 (MDCCCIV) was a leap year starting on Sunday of the Gregorian calendar and a leap year starting on Friday of the Julian calendar, the 1804th year of the Common Era (CE) and Anno Domini (AD) designations, the 804th year of the 2nd millennium, the 4th year of the 19th century, and the 5th year of the 1800s decade. As of the start of 1804, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.