1678 (MDCLXXVIII) was a common year starting on Saturday of the Gregorian calendar and a common year starting on Tuesday of the Julian calendar, the 1678th year of the Common Era (CE) and Anno Domini (AD) designations, the 678th year of the 2nd millennium, the 78th year of the 17th century, and the 9th year of the 1670s decade. As of the start of 1678, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.