1660 (MDCLX) was a leap year starting on Thursday of the Gregorian calendar and a leap year starting on Sunday of the Julian calendar, the 1660th year of the Common Era (CE) and Anno Domini (AD) designations, the 660th year of the 2nd millennium, the 60th year of the 17th century, and the 1st year of the 1660s decade. As of the start of 1660, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.