1651 (MDCLI) was a common year starting on Sunday of the Gregorian calendar and a common year starting on Wednesday of the Julian calendar, the 1651st year of the Common Era (CE) and Anno Domini (AD) designations, the 651st year of the 2nd millennium, the 51st year of the 17th century, and the 2nd year of the 1650s decade. As of the start of 1651, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.