1684 (MDCLXXXIV) was a leap year starting on Saturday of the Gregorian calendar and a leap year starting on Tuesday of the Julian calendar, the 1684th year of the Common Era (CE) and Anno Domini (AD) designations, the 684th year of the 2nd millennium, the 84th year of the 17th century, and the 5th year of the 1680s decade. As of the start of 1684, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.