1781 (MDCCLXXXI) was a common year starting on Monday of the Gregorian calendar and a common year starting on Friday of the Julian calendar, the 1781st year of the Common Era (CE) and Anno Domini (AD) designations, the 781st year of the 2nd millennium, the 81st year of the 18th century, and the 2nd year of the 1780s decade. As of the start of 1781, the Gregorian calendar was 11 days ahead of the Julian calendar, which remained in localized use until 1923.